FIRST, A WORD in defense of the administration's arithmetic. There's a lot of rebellious grumbling in Congress about the warnings that the White House wants further cuts in the budget for the year beginning next month. Some congressmen, not all of them Democrats, suggest that the idea of a second round of cuts is a new and dismaying development.

They may well find it dismaying, but it certainly isn't new. When Mr. Reagan published his budget last March, he observed that he was going to need about $50 billion in savings to get down to his spending target. He hoped, originally, to pick up some of it through a drop in interest rates. But it was apparent last spring that his hopes were misplaced. The reconciliation bill--the omnibus budget cut passed by Congress in July--will reduce spending $35 billion next year. That leaves another $15 billion or so that's still to be squeezed out, even under the administration's original optimistic assumptions.

But it's also been clear for months that the administration's original economic assumptions were much too optimistic. Now for a word in defense of the Congressional Budget Office's arithmetic. The CBO made something of a stir last week when it observed that the administration's spending figures for next year are too low by about $20 billion. But again, that's not a new figure. The CBO first did the computation last March and found that the underestimate was $22 billion. The latest reckoning merely confirms the earlier one, and reminds everyone that there's still that $20 billion discrepancy hanging over the administration's totals.

Mr. Reagan's target for the deficit next year is $42.5 billion. But without those further savings, the budget is now on a track much more likely to produce a deficit of about $80 billion. That's $20 billion higher than the deficit for the year now ending, and it's the prospect of a widening defict that frightens the people who worry about inflation. Some of these savings can doubtless be accomplished by the administration alone. But anything substantial will require legislation.

None of these numbers is new. They have all been in circulation since at least last spring. Congressmen who now treat them as a great and dreadful surprise will invite suspicion that they were not paying attention earlier. But while the numbers have been available for months, perhaps it's fair to say that the implications are only now sinking in.