THAT PRIVATE lunch at the Capitol the other day was the formal beginning of the search for a compromise, bipartisan tax bill. The four men who attended were the senior Republicans and Democrats of the House and Senate tax committees, and their meeting was a heartening indication of future direction. The administration has recognized that it cannot expect passage of its original proposal -- the 30 percent cut in income tax rates over three years. It is based on economic ideas that few people in Congress have embraced and is not well adapted to a moment in which the country is trying to end inflation while simultaneously stepping up defense spending. The Reagan tax cut is too big, it is too broad, and it tries to reach too far into a profoundly unpredictable future.

There's a good case for a one-year tax cut to offset some of the recent bracket creep -- the process by which inflation pushes people into higher tax brackets. But if it is effective on July 1, as Mr. Reagan proposes, the budget deficit this year will be larger than last year's. That could only be harmful to Mr. Reagan's larger proposes. If he cannot contain the swelling of the deficit in his first year in office, not many people will believe that he can do it in his later years. The effective date for the personal tax cuts ought to be no earlier than October, and perhaps as late as next January.

How large a cut? Mr. Reagan's plan would reduce income taxes $44 billion in fiscal 1982. It would be wiser to cut about a third less. As for the later annual cuts that he has urged, that is a question to be left to the next Congress. This Congress has carefully decided not to take up this year's tax bill before it has actually voted on this year's spending, and that's not a bad principle to follow for 1983 and 1984 as well. Our own guess is that, after this year's budget is finally enacted, there isn't going to be much enthusiasm left for further large reductions in spending.

The administration has been arguing that lower marginal tax rates will provide the crucial incentives for stronger performance of the economy. That claim has never been plausible except, perhaps, for the highest bracket. It's time to bring the top rate down from 70 percent to 50 percent, if only to have done with the wild growth of tax avoidance schemes and the useless investment that it encourages. A top rate of 50 percent would automatically mean a maximum tax of 20 percent on capital gains, a level that is hardly onerous by historical standards or any other. If Congress wants to provide further encouragement to saving, it might consider attacking the bias in the present law that provides unlimited deductions for interest paid out by borrowers, while fully taxing the interest earned by savers.

As for the business taxes, the administration's plan is much too hasty and heedless. It provides far too much help where it's not needed -- to commercial real estate speculation, for example -- and not nearly enough to hard-pressed manufacturing industries.

The purpose of economic policy this year is, or ought to be, the steady reduction of inflation without the undermining of the country's basic social responsibilities to its people. That means a tax bill designed to lighten part of the load imposed by recent inflation -- but not one that tries to commit the government to future budget cuts that in good conscience it cannot make. Congress is now beginning the work of writing tax legislation that will support Mr. Reagan's goals more directly and surely than Mr. Reagan's own bill can hope to.