It doesn't sound that complicated, the continuing resolution that was the subject of the dispute between President Reagan and Congress yesterday.

Since no regular appropriations have been passed by both houses and signed into law for the executive branch, the resolution says executive agencies may operate until July 15 at:

* The level approved by the House;

* The level approved by the Senate;

* Or the level approved by House-Senate conferees, whichever is lowest.

In addition, the resolution provides that almost all domestic programs subject to the congressional appropriations process be cut back by about another 2 percent.

But these seemingly simple formulas produced a budget dispute so complex that probably no one in Washington could swear precisely what the resolution entailed.

House Democrats said they had given President Reagan the spending totals he wanted. But the president, taking his numbers from his rehabilitated budget director David A. Stockman, decided Congress hadn't cut as much as claimed and announced yesterday he would veto the continuing resolution if it reached him.

Senate budget aides complained that the White House and OMB were picking numbers to suit their political purposes. "We didn't finish writing the continuing resolution document until 10 a.m. this morning," an exhausted Senate aide said yesterday, "and we didn't have copies available until 2 in the afternoon Sunday ." There was no time to make accurate estimates of spending, he said, and "everybody counts the beans a different way."

"There are a lot of numbers floating around up here and we have to sort them out," Max L. Friedersdorf, the president's chief lobbyist, said as the House and Senate bargaining ended early Sunday morning.

One reason the numbers failed to add up was that so many separate programs are involved. A separate calculation had to be made for each, and there was no quick, across-the-board method for pricing out the resolution.

A second problem was a fundamental case of apples and oranges, administration and congressional aides agreed.

The continuing resolution is expressed in terms of budget authority, whereby Congress gives agencies the right to spend specific amounts in fiscal 1982, which began Oct. 1.

But what interests the administration most is what the agencies will actually spend. That depends on all sorts of factors: the state of the economy, for example, or the regulations governing a specific program or the kinds of projects in prospect. Throughout the weekend, as House and Senate conferees settled differences on various departments, OMB officials made calculations on what actual spending would be, based on the budget authority totals. OMB's experts and Congress' experts simply disagreed, Senate aides said.

A third critical problem arose over how to distinguish between discretionary non-defense spending--subject to the appropriations process and the 2 percent cut--and entitlement spending, as for veterans or Medicare benefits, which go up more or less automatically with inflation and population.

OMB officials accused House budget writers of making unrealistic cuts in entitlement programs by providing insufficient amounts of money to meet expected demand between now and July 15. "They low-balled the entitlements," said OMB spokesman Edwin Dale. "We regard those as non-cuts."

Some congressional aides said OMB had been arbitrary. "There is $22.7 billion for agriculture programs in the continuing resolution," one aide said. "OMB and the Congress didn't even agree on what part of that is entitlements."

Administration aides said Reagan's decision to oppose the continuing resolution was strongly influenced by the changing nature of this budget device.

Conceived as a stop-gap measure to give Congress a few more weeks to complete work on the regular appropriations bills, the continuing resolution has taken the place of the appropriations process in many cases, administration officials complained.

The departments of Health and Human Services, Labor, and Education operated throughout fiscal 1981 on continuing resolutions, and forcing the president to veto one of these measures with the government on the brink of shutdown is an encroachment on executive authority that Reagan won't tolerate, Dale said.