IF THE economic summit at Colonial Williamsburg follows well-established patterns, there should be more than enough of what Adlai Stevenson once called the ingredients of a diplomat's social life: protocol, alcohol and Geritol.

But while all three can keep things going, only protocol can guarantee that they'll remain friendly.

"You could spend a lot of time pulling out your hair without it," said the United States' chief summit organizer Michael McManus. He and his boss, White House Deputy Chief of Staff Michael Deaver, have spent months wrestling with the not insignificant task of making sure seven highly placed foreign noses go home in the same unjointed state as they arrived.

And, when the Americans learned a week ago that British Prime Minister Margaret Thatcher planned to go home in the middle of the summit to continue her campaign for reelection, McManus & Co. hauled out their seating charts again. To keep down confusion, each revision has been dated.

"When you lose someone you start all over again," said the unflappable McManus. "Protocol is a ready-made answer to all your problems."

McManus and others, including White House social secretary Gahl Hodges and Chief of Protocol Selwa Roosevelt, had to reseat everybody at tonight's dinner in the Governor's Palace, tomorrow's luncheon at Bassett Hall and tomorrow night's official windup dinner at the Abby Rockefeller Folk Art Center.

McManus also had to rearrange seating at the 36- by 12-foot custom-made mahogany conference table for tomorrow's plenary and joint statement sessions. With Thatcher literally out of the picture, things worked out better than might have been expected.

"We wind up with seven people at that table so that when President Reagan delivers the joint statement, the balance is prettier," said McManus.

Last night, President Reagan was to be the host of the first dinner at historic Carter's Grove plantation on the banks of the James River. Although dinner was to be a North Carolina-style barbecue of catfish and hush puppies, it was nevertheless fancy enough to be served in the dining room, and protocol dictated that French President Franc,ois Mitterrand sit on Reagan's right and Thatcher on Reagan's left.

Figuring it all out reminded one planner of an eight dimensional Einstein-esque physics problem set to musical chairs. The end result, however, is that five foreign leaders will sit next to Reagan once, while Mitterrand and Trudeau will be accorded that honor twice during the summit.

There is no official U.S. government publication on the subject of protocol. The flexibility protocol provides has another side to it; you can write the rules to suit to occasion.

A 19th-century writer on law and diplomacy called protocol "the code of international politeness."

Secretary of State George Shultz said more recently: "If protocol goes well, nobody notices it. If it goes wrong, all attention is diverted to that."

Said Roosevelt: "Thank God for protocol!"

And from McManus: "My only comment--it's a blessing."

With those thoughts in mind, here's how order of precedence has been determined for this weekend.

Reagan and Mitterrand, the only other head of state present, outrank Thatcher, a head of government, even though she has been in office longer than either. Canada's Pierre Trudeau is No. 4 in the order that finds West Germany's Helmut Kohl, Japan's Yasuhiro Nakasone, Italy's Amintore Fanfani and the European Communities' Gaston Thorn bringing up the rear. Though it has no bearing on this weekend's protocol, Trudeau holds the attendance record at economic summits: seven out of the nine since they began in 1975.

When it came to finding beds for everybody, summit organizers again substituted protocol for prayers. McManus factored in square footage, appearance of Colonial Williamsburg's 18th-century VIP guest dwellings and their gardens. On that basis, President Reagan got Providence Hall, a mini-estate, and Mitterrand drew the elegant two-story brick Lightfoot House on Francis Street.

At the other end of the street, by coincidence nearer Reagan, Thatcher was assigned the white-frame Chiswell Bucktrout House. Thatcher's bed, in fact, turned out to be a reproduction of one slept in by one of her colonial countrymen at the Governor's Palace two centuries ago.

However, unlike the changes in seating, McManus said summiteers weren't to move up a house with Thatcher's change of plans. "We're not asking anybody to repack their suitcases." Staying put will be Trudeau in Bracken House, Kohl in Moody House, Nakasone in the same Williamsburg Inn suite that Emperor Hirohito occupied in 1975, Fanfani in Orrell House and Thorn in Lewis House.

Reagan was to bring his own stewards, but Williamsburg Inn was to provide two-person teams for 24-hour service to each leader. Thessolonians Judkins, who has trained the teams over the past six weeks, said his men know the correct forms of addressing the visiting dignitaries and what their special personal needs are. Even the smallest details have been considered.

"We've told them not to put ice in the water because a lot of people from Europe don't like ice," said Judkins, who has worked for Colonial Williamsburg for 47 years.

Money may talk at the summit meetings, but hard cash for tipping is taken care of by the State Department. Instead, VIPs often give gifts to the house staff.

Judkins said former West German chancellor Helmut Schmidt gave him a butane cigarette lighter that he liked a lot--except that he doesn't smoke; and the emperor of Japan sent him "a nice piece of silk" as a thank you.

Probably nowhere has protocol been followed more rigidly than in the pomp of arrival and departure ceremonies. Following reverse order for arrivals, Thorn, as low man on the totem pole, was scheduled to be the first to helicopter into Market Square from Langley Air Force Base yesterday. On Tuesday morning, he can sleep late since Mitterrand is scheduled to leave first. President Reagan, by all that protocol dictates, should go first but he's sticking around to wave goodbye to his guests.

Nothing has been intentionally left to chance. Like all good supporting casts, Selwa Roosevelt and other participants went through hours of pre-summit rehearsals. Yesterday, when she was scheduled to climb in first for the short ride carrying her and each arriving leader to the Governor's Palace to meet President Reagan, it was neither by accident nor in deference to her sex.

By following her, Thorn and the others, scheduled to arrive at 30-minute intervals, would wind up on the right. That would permit Roosevelt to exit left and be on the ground to present each visitor to the president.

"What the chief of protocol is responsible for is form, not substance," said Roosevelt, playing down her role in the three-day extravaganza.

Her office and its half-dozen protocol officers have been heavily involved behind the scenes, helping McManus and the White House deal with the mega as well as the minutiae. Example: advising which initials in the "alphabet soup" behind a VIP's name can be dropped when there isn't enough room to engrave the whole title on a summit gift.

The eight summit leaders are dining together at two dinners, independent of their economic and foreign ministers and their sherpas or personal representatives. That's partly due to what summit organizers like to call the "unstructured" nature of the conference, but also because space is limited in dining rooms at Governor's Palace and Carter's Grove.

Even though he's the host, protocol dictates that Reagan be served first. And there is a strong possibility that the ubiquitous after-dinner toasts may fall by the wayside because of the complicated logistics.

When the Americans alerted the delegations that there would be one al fresco luncheon and if the day was hot, leaders might end up eating in their shirt sleeves, the word got garbled in transmission to "short" sleeves. One country replied that its leader was not accustomed to dining in short sleeves.

And as another indicator of the importance of protocol, there's been more secrecy surrounding summit food, gifts and conference tables than there has on summit business.

Colonial Williamsburg craftsmen made the official gifts that the Americans are giving their guests. Anyone disclosing the details, however, is rumored to face imprisonment in Colonial Williamsburg's Publick Gaol.

And until somebody leaked it earlier this month, equally secret was information about the two mahogany conference tables that the Kittinger Co. of Watertown, N.Y., designed and completed in a record five weeks. Weighing 2,400 pounds and with a total surface of 340 square feet, the perimeter of the larger table is 80 feet. The smaller leather-topped table measuring 16-by-6 feet will be used once, for a meeting today, in the House of Burgesses.

Costing $16,000 and paid for out of the summit's $8 million budget, the tables represent "the patriotic spirit" of companies like Kittinger, said McManus, which have responded to the administration's calls for private initiative involvement.

"Normally, those two tables would have cost us around $75,000," said McManus.

What happens to the tables when the summit ends is still uncertain. One suggestion is that the United States go into the rent-a-table business.

"With a conference going on somewhere in the world all the time, who knows, we might be able to clear up our national debt," said a summit observer.