IF YOU HAVE NOTHING else to do for the rest of August, you could always set yourself the task of figuring out who is right and who is wrong about the figures President Carter used to justify his veto of the $36.9-billion weapons-procurement bill. Mr. Carter announced at his last press conference that he was vetoing the measure because it authorizes a $2-billion nuclear-powered aircraft carrier and generates the money to pay for it by making cuts in other military projects he considers essential to the national defense. So far so good. It's only when you wade out into the figures he cited to illustrate these cuts that you find yourself waist-deep in the accounting Big Muddy.

We are prepared to stipulate that the president, in his prepared press-conference statement, made a hash of the numbers. He spoke of cuts that Congress was making in military appropriations bills still being worked on as if they were part of the completed authorization bill he was about to veto. And his version of both the size and impact of some of those cuts, whether in the authorization or the appropriations legislation was highly colored and in some respects misleading. It is too bad - because the argument over these considerations, some of which are really arcane, distracts from the central fact. It is that the president is right. The nuclear carrier, which would be the Navy's fifth, should not be built.

This is not a radical view. It is held by people in the Navy as well as by people in Congress who could by no stretch of the most perfervid imagination be called unilateral disarmers or even, to use the silly locution, "doves." The argument concerns the kind of Navy that is needed to meet the military challenges and threats likely to arise in the decades ahead. And the nuclear-powered carrier - huge and supercostly - is favored by those who, in the main, reject the claim that what is needed is a larger fleet of smaller craft, as distinct from a smaller fleet made up of giant floating targets. In fact, the nuclear-powered carrier that Congress just authorized had a certain just one-more-for-the-road aspect to it. It was to be the last of these ships. And even as such it was one more than Jimmy Carter, Gerald Ford, Harold Brown and Donald Rumsfeld had said was necessary.

The Carter administration, it is said, by failing to include some small starting funds in the FY '79 budget for a non-nuclear alternative, more or less invited the unwanted inclusion of the nuclear one, since it did not seem to be going forward with the substitute Congress had expected. Another tactical question concerns the wisdom of the veto now in view of the fact that the authorization measure the president vetoed does include some significant starts on the new Navy the administration has in mind, and some of these were apparently bargained into the bill at the price of including also the fifth and last nuclear-powered carrier. The fear is that Mr. Carter's veto will cost not only the offending ship, but also the steps toward a more mobile, efficient and cost-effective Navy that were bought with it. For some critics of the nuclear-powered carrier - not just for its proponents - all this, plus skepticism arising out of the confused figures Mr. Carter produced in his initial statements, adds up to an argument against the veto and for a congressional override.

We sympathize with much of what they are saying, but on the ultimate conclusion, we think they are wrong. The big, basic fact is that Congress has authorized a $2-billion expenditure for a ship that should not be built. We have not mentioned a related concern on the part of numerous people on the Hill and elsewhere that Jimmy Carter (the leaks and murmurs suggest this to be the case) undertook this summary, abrupt and rather unexpected action largely for purposes of - what else? - image repair.That may be the case, but if it is, we have no objection. As the president is, in our judgment, right and deserving of support on the merits, we think it is perfectly dandy that he heas chosen to stake some large measure of his personal political reputation on sustaining his veto. We are glad he has engaged his fortunes in this fight. We only hope he will conduct it with more efficiency and care than he showed on the way to the veto.