For the 310,000 Washingtonians who must make out the District of Columbia's complicated income tax forms, help is one the way -- but it won't arrive soon enough to affect this year's returns due this year on April 16.

Most witnesses at a hearing of the City Council's Finance and Revenue Committee endorsed legislation yesterday that would let taxpayers copy their income and deduction figures directly off the federal form, then calculate the tax they owe the city. That is the way it is done in Maryland and Virginia.

As it is now, under a District of Columbia law largely unchanged since 1939, the city's definitions very so greatly from federal tax law that taxpayers must compute a largely different set of figures.

"The D.C. form is much more difficult to fill out than the federal form," council member Betty Ann Kane (D-At Large) observed.

As an example of the city's differences, Auditor Matthew S. Watson testified, donations to the United Negro College Fund -- whose forthcoming campaign was endorsed this week by Mayor Marion Barry -- cannot be deducted on D.C. tax returns. Only charties that operate in the District qualify.

Yesterday's hearing was held on a bill by Hilda Mason (Statehood-At Large) that would let District taxpayers figure their local taxes as a percentage of federal taxes. The federal government would do the collecting.

Kenneth Back, director of the D.C. Department of Finance and Revenue, testified that such an approach would let Congress decide the city's tax policy -- as it did before home rule -- and could cause fluctuations in revenues as the federal law is changed.

John A. Wilson (D-At Large), chairman of the council committee, said he would support the approach used by Maryland and Virginia that has been recommended by Mayor Barry and the city's Tax Revision Commission.

No action was taken at yesterday's hearing.