The question is not so much "how sharp to kill"?... a BH doesn't need to be sharp at all in order to kill. The reason many of us are fanatical about the level of BH sharpness has more to do with game recovery. All else being equal, the sharper the head, the better the bloodtrail will be and the better our chances of getting the animal loaded in the truck will be too. (which is the real goal, right)? Stone (knapped) heads are wicked sharp BTW and as my buddy Jake always says: "I never lost an animal because my BH was too sharp".
Ron