Most of the 125,000 government managers/supervisors moving (many nervously) into a new "merit pay" world will love it -- once the raises are handed out -- according to the people who designed the competitive salary system.

Merit pay is one of the corner-stones of President Carter's civil service reform act. It will cover seven of every 10 employes in pay Grade 13 through 15. Salaries for these people, now being picked by individual agencies, range from $29,375 to $50,112.50.

Unlike most civil servants who will continue to get regular automatic pay raises each October, the "merit pay" people are guaranteed only half the regular percentage increase set by the president. To get more, they must get good grades -- called performance appraisals in government -- from their bosses.

GS 13, 14 and 15 people destined for the merit pay system -- perhaps 65,000 of them here in Washington -- no longer will get longevity pay raises and incentive awards that will continue to go to rank-and-file workers.

In place of automatic increases they give up for merit pay, the supervisory/management people will compete for raises from money available within individual agency merit pay cash pools.

Officials in charge of merit pay say that Friday's column on the subject made it appear that the GS 13 through 15 people are entering a dog-eat-dog arena. Rather, they say, most people under merit pay stand a good chance of getting at least the full amount of each October's increase. Many, they say, will get more. Some will get double the amount. They explain it like this:

Suppose this October's raise is 6.2 percent -- the amount Carter has budgeted. Most federal workers below Grade 13 will get the full amount.

Individuals under the merit pay will get only one-half, or 3.1 percent, automatically. Funds for merit raises available to them will include the full amount of the 6.2 percent raise plus an extra amount -- and 2 percent per employe in some agencies -- from funds "saved" by denying the managers/supervisors incentive and longevity increases. Money from that "savings" will amount to an additional 2 percent, or thereabouts. It will be added to the merit pay pool. That would make it about 8.2 percent potentially for everyone under merit pay.

Supervisors/managers rated as less than satisfactory will get only half, or 3.1 percent. Those rated satisfactory very likely will get the full 6.2 percent. Those rated better still will get higher raises, depending on how much money is in each agency or department's merit pay pool.

How much more the better-than-satisfactory people get depends on how much money is in the pool, how it is divided and who divides it. If most people are rated "satisfactory" they will get slightly more than the regular 6.2 percent increase. If large numbers are rated unsatisfactory -- getting half raises -- there will be more money in the pool for people with the best ratings.

A couple of agencies, Office of Personnel Management Health and Human Services (formerly HEW) and the Small Business Administration will begin merit pay this year. Others will have it ready for the October 1981 pay raise.

Most agencies will not make merit pay adjustments until after the regular October increase. That means individuals who will get more under merit pay will get it later, perhaps in November or even December. The increases will be retroactive to October.