THERE IS NO hotter question in Washington than what the instructions will be that President Reagan gives to the team he is about to send back to Geneva to resume negotiations with the Soviet Union on reducing intercontinental strategic missiles. That the instructions for the START delegation will be changed is clear. Explicitly and implicitly, Mr. Reagan accepted a requirement to make his START proposals consistent with the recommendations of the Scowcroft commission. It is, we think, a political reality that Congress stands ready to hold him to his word: it has control of the MX money valve. But how will the START proposals be changed?

There's an intense argument among the conservatives of the Reagan national security team. It centers on missile lifting power or throw-weight. Historically, Moscow has built bigger missiles, and so it has a theoretical advantage of better than 2 to 1 by this measure. In the administration's first START proposals, tendered when the negotiation opened in May 1982, the United States contemplated a first phase in which the throw-weight disparity would be addressed though the emphasis would be on making deep reductions in numbers of warheads and missiles. In a second phase, emphasis was to be put directly on achieving equal throw-weight levels.

The question now is whether, in revising the START proposals in accordance with the Scowcroft recommendations, to maintain the two-phase approach or to combine the phases and demand right off that the Soviets come down to the American throw-weight level.

The argument for continuing on the old track is that it would improve the chances for an agreement, since it is assumed that any large new demand on Moscow would not be well received; this is believed to be true in part because Washington will be submitting to Moscow some other, smaller START changes that are necessary and desirable. The argument for switching and putting greater priority on throw-weight is that without equal limits the Soviets could break out and quickly add many warheads to existing missiles if things went altogether bad.

Mr. Reagan is being asked the quintessential arms control question: whether to take a relatively cautious course that is likely to facilitate (though not ensure) an agreement with Moscow, or to try to protect against an extra range of dark future contingencies by advancing proposals that will make it harder to get an agreement. Even if the president were not under political pressure to take the first course, all we have read and heard about it, from opponents as well as those in favor, makes it sound to us a right, safe and reasonable thing to do.