It depends, but A4=440Hz seems to be the standard assumed by just about every electronic instrument I've encountered (and while some fancier ones make that conf

It depends, but A4=440Hz seems to be the standard assumed by just about every electronic instrument I've encountered (and while some fancier ones make that configurable, most don't). Realistically, the "correct" reference pitch for a given performance is going to depend on environmental conditions, since temperature and humidity will affect the "normal" pitch for an instrument (and will do so in different ways depending on the type of instrument). Most pragmatic approach would be to base it on the hardest-to-retune instrument in a given ensemble (i.e. take a tuner to it while it's playing some octave of A, do the requisite multiplication/division to get to A4, and there you have your reference pitch). Speaking from experience in marching music, the resulting reference pitch tended to vary between 438Hz and 442Hz, with very rare exceptions.