when your talking anything below 1/4-20 taps the tap itself is one of the biggest factors.
what tap are you using?
probably the biggest factor is drill size (and you should be fine with a #24) Since most taps deform the aluminum you can actually go to a #23 or 5/32 then check with a go no/go gauge they will probably still give you good numbers.
You just have to be sure you QC the thread properly if you use oversize drills to make sure it gets deformed into spec.
I typically use ex-o taps (I think OSG makes them), they aren't cheap but the are the real deal. Emuge makes really nice stuff, but, I haven't used it much since Ive been doing 90% aluminum these days.
you can peck drill if you want, but, I tap 10-32 to .75" in one shot all day long.
thanks
Michael T.
"If you don't stand for something, chances are, you'll fall for anything!"