IF YOU RECEIVED a W-2 form from your employer last week and are beginning to think about your federal tax return, you will be glad to learn that members of Congress have also received some useful tax information from the office. The leaders of both houses have consulted the legislative calendars for 1981 and have advised members of the exact amount they can deduct from their taxable income under the provisions of the congressional tax break law enacted just before Christmas.

That new law--made retroactive to cover all 1981 taxes--allows members to deduct $75 per day for every day of the year except those included in a recess period of five days or more. The magic number for the House last year is 262 days, and for the Senate, 256 days. This means that members of those bodies can deduct $19,650 and $19,200 respectively from last year's income without proving any business expenses at all, and without being subject to IRS audit on those deductions. If members of Congress can prove that living expenses in Washington exceeded these amounts, their deductions could be much higher. There--doesn't that make you feel better as you sharpen your pencils and line up your 1040 forms?

Perhaps the only members of Congress who won't be happy about the new law are our own representatives in the metropolitan area who live in their districts and do not have a second home here in Washington. IRS attorneys say they will not be able to take the new deduction since they do not have reasonable and necessary business expenses away from home. The inequity of granting the enormous tax bonanza to all members of Congress except the half- dozen or so who already live here is apparent. As we said in this space last week, a reasonable and uniform increase in salary is far preferable to the uneven benefits available through the tax system. Sen. William Proxmire has announced that he will introduce legislation to repeal the tax break when Congress reconvenes. Now that everyone has had a few weeks to examine the new law, to look at the dollar figures involved and to evaluate the inequities inherent in the tax approach, Mr. Proxmire should have widespread support for his proposal.