there are TONS of factors, so there really is no "one" way to setup a bike, rider weight, tire size, swinger length, type of motor work, carbs, pipes, all play thier own parts in determining what gearing to run, plus every track will be differant as well, if its a tighter track your gearing will be differant then a wide open track, and a loose dirt track will be differant then a hard clay track...
You best best is goig to be to get a few differant gears and just test them out.