Bill,
I usually use one thread diameter or 1/2 the head diameter if that's a
consideration
for a minimal spacing, but you can fudge that a little.
RichD, in DRY Atlanta (Spit in this direction. Anything will help)

In such a situation, how close to the edge of an Al part would you
consider it "safe" to tap for 4-40 and 8-32, respectively?