The Reagan administration has developed hypothetical timetables for a 50 percent reduction in U.S.-Soviet long-range nuclear weapons that would require Moscow to dismantle most of its 308 large, SS18 intercontinental missiles in the earliest phase to ease U.S. fears about Soviet first-strike abilities, administration sources said yesterday.

The schedules, including "reductions on quantity and category" of weapons, were worked out as part of earlier U.S. arms reduction proposals, sources said.

A five-year reduction plan for both superpowers was developed with the 1982 strategic arms reduction talks (START) proposal to reduce strategic missile warheads by about one-third, one source said.

U.S. officials, in working out these earlier arms-control reductions, found that as U.S. land-based missiles were reduced, the 10-warhead, highly-accurate SS18 ICBMs became more of a first-strike threat to the remaining American missiles, sources said.

Therefore, one official said, it became necessary for reduction formulas to "have the multi-warhead, land-based missiles among the first to go."

Soviet Party leader Mikhail Gorbachev's comprehensive arms proposal made public Wednesday called for each side in the first five to eight years of the agreement to reduce strategic missiles and bombers by 50 percent, without specifying how those reductions would be made. The Gorbachev plan only called for each side to be left with missiles and bombers having a capacity to carry 6,000 warheads or bombs.

A Soviet diplomat said yesterday that under the Gorbachev proposal, each side could make its own choices on how it wanted to reduce. "There would be freedom to choose" among various nuclear systems, he said. But, he added, "this is a matter that would be negoti- ated."

U.S. officials pointed out yesterday that this would permit the Soviets to keep all the SS18s until the second phase of the Gorbachev reduction plan, which would eliminate the remaining strategic weapons by 1997.

Secretary of State George P. Shultz, in an interview Thursday on the "The MacNeil/Lehrer NewsHour," made clear that the United States would present its own plans for dealing with the reduction schedule.

"What we have to do, if we're going to get out a schedule," Shultz said, "is find a way to come down so that the security and deterrent capability of each side is main- tained."

Shultz was referring to the U.S. need for certainty that at any point in the reduction, the U.S. retaliatory missile force "would be no less invulnerable than they were before the reductions started," one State Department official said yesterday.

Shultz also said that the Soviet plan for allowing each side to choose the weapons-reduction schedule "would have us come down in a way that wouldn't seem to reduce their strongest weapons very much and would cut into ours."

With the White House special arms control group (SAC-G) still involved in its initial detailed review of comprehensive Gorbachev proposal, officials said yesterday it was still far too early to speculate on when possible U.S. responses would be ready.

Asked specifically about the missile-reduction issue, one senior official said, "That's a little like working on setting the date and place for a wedding without first picking who you want to marry and whether she will accept."

In a related matter, arms control advocate groups yesterday called the Gorbachev proposals a forward step in ending the arms race and praised President Reagan for not immediately turning it down.

Former CIA director William E. Colby, speaking at a joint news conference of the Center for Defense Information, SANE, the Nuclear Weapons Freeze Campaign, the Committee for National Security and Citizens Against Nuclear War, said, "I think we are grateful that he [Reagan] has accepted the idea and not rejected it out of hand."