For all the huffing and puffery that went into producing $130 billion in tax increases and spending cuts this week, President Reagan and Congress are less than halfway toward achieving their target of $380 billion in deficit reductions through 1985.

Both Congress and Reagan headed for home yesterday, but when they return after Labor Day they must contend with the rest of the cost-cutting. And some of that may be beyond their control.

The $130 billion in deficit reductions that Congress already has voted will have a multiplier effect: it will reduce federal borrowing needs and interest costs by an additional $15 billion to $20 billion.

But that leaves at least $230 billion to be nailed down if the deficits for the next three years are to be kept within the bounds Congress set in the budget resolution adopted in June.

More than half this $230 billion is within Congress' power to produce. It need only live up to the commitments made in the budget resolution.

Though total compliance is uncertain, most of these things will probably be done. But the rest of the savings -- principally another $80 billion counted on the assumption that all other deficit-reduction targets will be met and interest rates will fall in response -- cannot be taken for granted.

As much as $130 billion of the total anticipated three-year savings fall into the "soft" category, meaning they are not assured under the budget and its enforcement provisions, according to one unofficial Senate Democratic staff estimate.

This means that next year's official deficit projection of $104 billion, with lower figures to follow in future years, is in serious doubt. "Even with what we did today," said House Budget Committee member Leon E. Panetta (D-Calif.) after Thursday's vote to increase taxes, "I think we'll be approaching [a deficit of] $175 billion to $200 billion."

The problem in a way is that the economy is mightier than the Congress. Congress this week approved $98.3 billion in tax increases and $30 billion in spending cuts through 1985, enormous bills by historical standards.

But economic turns -- just fractional shifts in unemployment, inflation or interest rates -- can offset these congressional actions, and move billions of dollars in or out of the Treasury.

For these and other reasons, projected budget deficits are little more than educated guesses.

Without anything more than the tax increases and spending cuts voted this week, the projected deficits for the next three years would be $157 billion in 1983, $166 billion in 1984 and $175 billion in 1985, clearly more than Congress sees as politically or economically tolerable.

The $30 billion in spending cuts approved this week came almost entirely from so-called entitlement programs, the large basic benefit programs such as Medicare, Medicaid, food stamps and civil service and military retirement in which money goes out automatically to all who qualify (or are "entitled"). These programs are not subject to the annual congressional appropriations process.

Congress left to later the cuts it was obligated by the budget resolution to make in appropriations, including defense, and other so-called discretionary spending.

To get down the deficits down to levels projected in the budget resolution -- $104 billion for 1983, $84 billion for 1984 and $60 billion for 1985 -- there must be these further restraints on spending:

Congress has to come up with three-year savings of $26 billion in defense and $35 billion in domestic appropriations as it prepares its annual money bills -- or more likely a single gigantic "continuing resolution" for all or most agencies -- for the start of the new fiscal year on Oct. 1.

These "cuts," like the ones voted this week, may not reduce total spending. They are reductions from what experts estimate spending under existing law otherwise would be.

In its budget, Congress adopted target ceilings for each major category of spending. These will become binding ceilings next month if Congress has not adopted a second budget resolution in the meantime.

Congress then will be able to pass spending bills that breach the ceilings only by waiving its budget rules. Hence there is a reasonable although not certain chance that Congress will live within the limits.

The defense "cut" is actually a one-quarter reduction from the more than $100 billion military buildup that Reagan is proposing for the next three years. The domestic "cuts," with some exceptions, amount to a freeze based on fiscal 1982 levels of spending. For domestic appropriations, this means no additional money to offset inflation or additional caseloads for particular programs.

Another $26 billion in savings is assumed through imposition of a 4 percent cap on federal pay increases through 1985. This, too, is within the power of Congress and the White House as they set government pay levels for the next three years.

Reagan is being counted on for $47 billion in "management initiatives," or administrative cost-savings, that Congress will have little if anything to do with. In his budget, Reagan calculated he could save $3 billion by attacking "waste, fraud and abuse," $4 billion from improved debt collection, $9 billion from disposal of surplus federal property and $22 billion from accelerated leasing of the Outer Continential Shelf for oil and gas drilling.

There is considerable skepticism in Congress -- and some within the administration -- that all of these administrative savings can be achieved.

These and other doubts have prompted the Congressional Budget Office to suggest that even the target deficits in the budget resolution, including $104 billion for next year, are probably underestimated.

In the 3 1/2 weeks between the end of its Labor Day recess and its scheduled adjournment in early October, Congress will grapple with defense and domestic appropriations bills for fiscal 1983, including one-year cuts of $7.8 billion in defense and $5.9 billion in domestic programs.

But, with the new fiscal year starting on Oct. 1, few if any of the 13 appropriations bills are expected to pass Congress, meaning that another huge "continuing resolution" to fund the government on a stopgap basis is more than likely.

So is a veto confrontation between Congress and the White House if Congress fails to tailor its spending plans to conform to Reagan's interpretation of budget priorities.

Reagan has twice vetoed one supplemental appropriations bill for the current year and has threatened to veto a second one. The expectation is that he may crack down hard on budget-busting, especially on the domestic side, by liberal use of the presidential veto.

As for the future, Office of Management and Budget Director David A. Stockman has said further spending cuts, especially in entitlement programs (by implication including Social Security), are going to be necessary next year.

Reagan has said he will be bound by deficit targets in future years, but not necessarily by the defense limitations that Congress imposed on him, meaning a likely further squeeze on domestic programs.

So the budget surgery, which began when Reagan took office, is far from over. And, to the extent that Social Security is next to go on the operating table, the pain may have only begun.