Never thought of a formula like that, but I think that's pretty sound for most people. For hunting purposes, I'm pretty happy with 2" groups at 20 yards, 3" at 30 and 4" at 40. I recently mentioned on another post that I'm turnng in softball-sized groups at 50, which I said is probably not world-class, but I think it'll be good enough for my upcoming elk hunt. So ... all in all, I think Ed's right on.