You are generally safe if the taper is held under .0002 total variation for the length of the taper. Much more variance than that and it is no longer a self-holding taper. This is for hardened and ground fitments. If you are turning a temporary piece, leave the finish a little rougher and keep it unhardened, and your "forgiveness" will jump up to about .002. Good luck. If you need to be extremely precise, you'll need to know your class of fit , x y z etc, and get ahold of ANSI.
