On Wednesday, President Reagan sent Congress his "Truth in Federal Spending Act," which would, in part, force Capitol Hill to report the true cost of legislation.

Yesterday, Sen. Lawton Chiles (D-Fla.), who chairs the Senate Budget Committee, doubted that the Reagan administration applied that standard to its fiscal 1989 budget.

In a letter to Office of Management and Budget Director James C. Miller III, Chiles questioned the budget's assertion that it includes a 4 percent increase in funds for programs aiding the homeless.

In fact, charged Chiles, an analysis by the budget panel shows that the president's budget would cut funding for the homeless by 52 percent, from the $792 million authorized in fiscal 1988 to $377 million.

"There's been a lot of fanfare on the homeless. And now they've whacked it," Chiles said. "It sounds like the same thing that happened to drug programs last year."

The administration's request to cut spending on the homeless came seven months after Reagan signed into law legislation authorizing about $1 billion for housing, food and health programs for the homeless. In his budget last year, Reagan proposed similar cutbacks in drug-abuse prevention shortly after signing legislation authorizing a major effort to combat illegal drug use.

Miller, who had not seen Chiles' letter, said, "I can't figure out how in the world he comes up with figures different than ours unless the {committee} staff used some sleight of hand."

Miller said it appeared that the committee staff had added unexpended budget authority from fiscal 1987 to the fiscal 1988 program level and then compared the total to the president's fiscal 1989 request. If the administration used that method on the defense budget, Miller said, "we would have cut the Defense Department in half."

"I'd be surprised if Congress turns its back on a program it initiated," said Chiles, predicting that Congress will provide more than the amount the president requested for the homeless. "It's not a problem that's waning."

Chiles said the proposed cuts would be devastating because they would come as the government's program of donating surplus food to the needy is ending because the surpluses are almost gone.

The Agriculture Department says surpluses of cheese, nonfat dry milk, rice and honey will disappear by May because falling price supports have made commodities more competitive on the open market.

Robert Greenstein, director of the Center on Budget and Policy Priorities, said the "situation even looks grimmer" because the administration has proposed to cut the program under which the government buys food for the homeless from $114 million to $80 million.

"It suggests that last year's effort on the homeless may be a one-shot deal," Greenstein said. "It was set up in fiscal 1988 only to have the rug pulled out from under it in fiscal 1989."