The Post's editorial "Competing for the Best Teachers" {Aug. 9} cited a beginning teacher's annual salary in the D.C public schools as barely more than $19,100 (for the school year, though it might have to suffice for 12 months). By an old standard that was also cited, one shouldn't pay more for rent than 30 percent of one's gross salary or, in this case, $500 per month ($475 is closer, and every penny counts).

I tried to rough out the after-tax income of that salary -- to cover 12 months -- and the best I could figure out was almost $14,400 per year, or $1,200 per month. This assumes basic medical and hospital insurance is provided by the city. A $475 rent with utilities included would be just short of 40 percent of that paycheck, after D.C., federal and Social Security (soon to be 7 1/2 percent) taxes. Another expense is passable furniture (could it be rented initially?).

A nice one-bedroom apartment at $600 per month, shared, would take 25 percent of the salary (and one could share rental of furniture, allowing nicer furniture). But sometimes sharing, even with the best of friends, is a disaster. A nice furnished room at $350 per month (about $81 per week) with kitchen privileges might not be cheaper, but would be a lot more tranquil and private -- and one would not be bound by a 12-month lease (in case one's landlady was a disaster).

I hate to mention this, but some of the 400 to 450 new teachers the District expects to need every year through 1994 owe on student loans for their college years.

Prince George's County and others have the right idea, and I bet they'd even let their teachers, cops, etc. live in the District if they wanted to. GERALD V. CARROLL Washington