Defense Secretary-designate Caspar W. Weinberger yesterday shied away from the defense spending standard to which conservatives have tried to hold the Carter administration the last few years: that the Pentagon budget rise each year some fixed percentage beyond inflation.

"I don't believe in these fixed percentages," Weinberger told the Senate Armed Services Committee, which, on the basis of the friendly questioning yesterday, seemed certain to confirm him.

The key question, the cost conscious Weinberger said, is not the percentage of dollar increases in future military budgets but "are we getting the strength we need?"

Defense Secretary Harold Brown was mauled by conservatives a year ago when he resisted Senate efforts to commit the Carter administration to raise the military budget 5 percent a year after inflation.In contrast, no senator challenged Weinberger's stated intention to remain flexible on the question of how much is enough.

Weinberger told reporters after the hearing that he was concerned about the impact fixed percentage increases would have on Pentagon budgets in the "out years," an apparent reference to the likely money requests for fiscal 1983 and beyond. President Carter will ask Congress for $196.4 billion for fiscal 1982, representing an after inflation increase of about 4.6 percent.

One reason the hawkish members of the committee did not challenge Weinberger may be because of the generally hard line he laid down about the Soviets generally and the Carter administration's strategic arms limitation treaty (SALT II) specifically.

"It does seem clear," Weinberger told the committee in talking about his view of Soviet intentions now that he has been briefed on their military activities at the Pentagon, "that it is their intention to proceed with an attempt to secure an imbalance that would make it very difficult, if not impossible, for us to assert our interests or the interests of our allies in any portion of the world. And I think that obviously this has to be totally unacceptable."

Weinberger pledged to work to redress what he said was an imbalance between U.S. and Soviet forces by putting existing troops and weaponry in better fighting condition and by pursuing nuclear weapons that would improve "the strategic balance."

The basic objective of the Reagan administration, Weinberg continued, will be to make the United States look so strong tha potential enemies would dare not attack and would think twice about such terrorist activities as taking Americans hostage overseas or otherwise harming them.

In discussing SALT II, which President Carter did not put up to a Senate vote last year for fear it would be rejected. Weinberger said that it would be "a good six months" before the Reagan administration will be ready to discuss a new agreement with the Soviets. It will take at least that long for the new administration to sort out its military options and decide which ones to pursue, he said.

This prospect of delaying any resumption of SALT talks for at least six month contrasts to an earlier prediction by Sen. Charles H. Percy (R-Ill.) that the discussions would get under way shortly after Reagan's inauguration.

"I think it's important that the process continue," Weinberger said of arms control negotiations with the Soviets. "But negotiations must proceed from a different kind of basis than those in the past. There has to be a clear signal that we are resolutely embarked" on the path to redress the strategic balance. "Sometimes a failure to get a treaty is a success."

In response to other questions covering a wide variety of subjects, Weinberger made these other points:

Draft registration -- Although Reagan opposed taking this step in pacetime, Weinberger said "there would be at the very least severe administrative problems in rolling back" the requirement that is now in effect. a

Conscription -- He said the all volunteer force, which took over from the draft in 1973, should be continued at least until the administration can assess the impact of higher military pay and improved benefits before resorting to conscription.

Persian Gulf -- "We will need a military presence there if countries there concur . . . We should get ourselves in position as quickly as possible to carry out commitments."

He said Carter, by declaring in a State of the Union address that the United States would go to war if necessary to protect its interests in the Persian Gulf, promised more than he could deliver militarily. Besides that, Weinberger said, Carter did not consult with interested countries ahead of time, making for an "extraordinarily clumsy and ill-advised" presentation.

While admitting that the United States is short of the military forces it needs to protect its interests in the Gulf, Weinberger said that admitting this reality does not represent "any watering down" by the Reagan administration of the commitment to protect interests there.

MX missile -- He said the new 10-warhead missile is needed, but " would want to examine a wide number of options" before recommending that the Carter plan to deploy 200 MX missiles in Nevada and Utah be pursued.