Florida Sen. Lawton Chiles, the Senate Budget Committee's ranking Democrat, contends that his Republican colleagues are pulling off a flimflam by claiming that their budget compromise with President Reagan would reduce military spending by $18.5 billion in fiscal 1986.

Chiles' complaint, which he said he will register anew on the Senate floor this week, provides insight into how the administration and its allies are using different starting points when they measure cuts in defense and domestic spending.

When Congress left for its Easter recess, Senate Majority Leader Robert J. Dole (R-Kan.) issued a press release, declaring that Reagan had agreed to a series of budget cuts, including one that would reduce military spending by $18.5 billion in fiscal 1986. Chiles scoffs at that figure, saying the cut amounts to only $5.8 billion if the same starting point for measuring proposed domestic reductions is used.

A Dole spokesman agreed that two different starting points were used, but said Congress has never been able to agree on a standard figure for the Pentagon budget.

Chiles said Dole wanted to make the Republicans' proposed Pentagon budget cut look big and the domestic one, small.

Isn't that just politics as usual, Chiles was asked. "Oh, if I thought I could do it and get away with it, I'd probably do it, too," he replied. But he added that it irks him that nobody seems to be indignant about "this double standard."

As his starting point, Dole used the figures Reagan agreed to last year in the so-called Rose Garden budget compromise. That budget called for $333.6 billion in budget authority and $294.6 billion in spending.

Chiles contended that the Congressional Budget Office figures of $324.7 billion in budget authority and $281.9 billion in spending are the more legitimate because Congress approved them last year when it hammered out budget ceilings for fiscal 1986. Both parties use the CB0 figures when measuring domestic cuts, Chiles added.

Dole said that Reagan has accepted reductions that would set military spending at $276.1 billion in fiscal 1986, or $18.5 billion less than the Rose Garden total and $5.8 billion under what Congress set as the target last year. If the CBO figures were used for both defense and domestic programs, Chiles contended, the public could see more clearly how the ax is only nicking defense, but cutting deeply into non-military programs.

"I practically took my clothes off in there," Chiles said of his effort to persuade the Senate Budget Committee to use the CBO figures for measuring the proposed defense and nondefense cuts.

"If you want us to come to the ball," Chiles said he told the Republicans, "let us dance on a level floor."

But the Republican majority voted to use two sets of figures, which means the argument will be renewed when the Senate takes up the committee's budget resolution.

Although Chiles plans to make his argument again, he said it may be easier to influence wavering lawmakers by reminding them of the bow wave of military spending that will hit the economy later in this decade.

The Reagan military buildup in the first four years has been so gigantic, Chiles said, that defense spending will reach all-time highs in the coming years as big bills for weapons already ordered come due. This will be true whether the fiscal 1986 budget is frozen at its current level, as the Senate Budget Committee has recommended, or grows by inflation plus 3 percent, as it would under the Easter compromise.