From my experiences, I've found BBBastard to be the best BB's (And I'm not just saying that because I'm selling them... I'm selling them because THEY are the best)
Outdoors: .25 will work for any FPS... .2 is really only good for chronoing, indoors and CQB.
0 to 250 FPS: .20
250 to 350: .25
350 to 420-450ish: .28
450 to 500ish: .3
Past 400 it's basically personal preference.