THE PRESIDENTS proposed changes in the way federal salaries are calculated won't do much for his popularity with civil servants. If adopted, they would eventually cut the amount of money the government pays its employees by about $3 billion a year. While that will be hard for civil servants to accept, the fact is that the Carter proposals go in the right direction. They are an obvious next step in the prolonged effort to make government salaries fully comparable to those paid by private industry.

The central, and most controversial, item in this new plan is that which would include the value of fringe benefits when federal and nongovernment salaries are compared. These benefits, estimated to run about 30 percent of the real cost of the government payroll, are not counted now.

It may have been a proper accounting technique to disregard such costs when the civil servants won their struggle some years back to make the government pay "comparable" salaries. But as the fringes - pensions, vacations, medical benefits and so on - have increased dramatically in value in both the public and private sectors, excluding them distorts "comparability" beyond recognition. Most employees in private industry began long ago to count the value of those benefits when comparing job offers from different employers.

The president's other big chance would be to break up the nationwide pay scale for white-collar workers. This would mean that prevailing local wage rates would be taken into account when federal salaries were set. Given the difference in the cost of living between Washington and, say, Plains, Ga., there is an argument for making government salaries reflect local conditions. Large private companies know they must pay more in Washington or New York than they pay in many southern and southwestern communities for the same quality of worker.

If Congress goes along with the president on all this, Mr. Carter will have fulfilled one of his major compaign promises. His civil service reform legislation, which passed last year, provided the government with some of the flexibility it needs to deal with a large, and sometimes blundering, bureaucracy. His pay proposals would go a long way toward quieting the constant (and growing) complaint from business organizations, state and local governments, and also ordinary citizens that federal salaries are too high. As unwelcome as the pay proposals may be to federl employees, eliminating the basis for that widespread complaint may be more important to their economic welfare in the long run.