I read with interest Fairfax County Board of Supervisors Chairman Audrey Moore's commentary {"In Fairfax, Fiscal Caution vs. Future Shock," Close to Home, July 29}. While calling for fiscal restraint is heading in the right direction, Mrs. Moore has not gone nearly far enough. She is merely talking about reducing the yearly rate of budget growth from more than 8 percent to about 4 percent. She should be talking about an overall decrease of at least 6 percent.

The day before Mrs. Moore's piece appeared, The Post's Real Estate section reported that Fairfax County residential real estate prices decreased more than 6 percent last year. How can Mrs. Moore plan for a 4 percent increase in the budget when the county's primary tax base is decreasing by more than 6 percent?

Politically Mrs. Moore and her fellow supervisors cannot raise the tax rate. They have a persistent and possibly slowly growing tax revolt among citizens. And as the sluggish economy continues to drag, and people feel this in their personal lives, this tax revolt may pick up many more advocates. In addition, timing for the supervisors is against any tax-rate increase. They will be discussing this budget just a few months before an election.

Population growth (i.e., more residential property to tax) will not match the projected 3.5 percent, and even if it does, this will be insufficient to offset a more than 6 percent loss in residential real estate values and tax revenue. The supervisors cannot reasonably expect to increase business taxes enough to offset this possible loss in residential real estate taxes.

Mrs. Moore and her fellow supervisors don't have much choice but to plan on reducing the FY-'92 budget by at least 6 percent. And by the time they start discussing this next budget, property values losses may be even greater, and they may have to consider and even greater overall cut -- maybe as high as 9 percent.

Budget guidance for FY-'92 is scheduled to go to county departments in August or September. Since this is going to be shock therapy for a county bureaucracy long accustomed to ever-increasing budgets, the sooner they are officially notified of this planning guidance, the sooner they can start squawking and screaming. And when they are through with that futile gesture, they can start planning on reducing much of the fat and waste in county government.

G. H. BUCHANAN Fairfax Station