I like to have my flints as folks have written, to sit just a fraction of an inch from the face when the cock is set at half-cock position. Bevel up or bevel down I determine by the sparking AND the chipping of the flint. If both ways spark well but in one position that particular flint chips a lot more, I will use the other position.
I have found for the very large musket locks, that a lead wrap works best in my muskets, but otherwise a leather wrap works very well in other guns and especially in rifles. For locks that for whatever reason don't seem to like a particular flint..i.e. the sparks are not so numerous and/or the flint chips heavily with the bevel in either position, I experiment with changing the flint angle just a tad. I have found that sometimes placing a small piece of wood from a small, used, wooden matchstick, under the back of the flint where the flint contacts the jaw-screw, will change the flint edge angle downward, and often give you good sparks but reduce the bashing of the flint edge.
In addition to Zonie's advice on checking the flint impact into the pan, you should check the forward corner of the flint where it closest to the barrel. Flints are sold sorted in different sizes, but those are approximations, and you might have a flint that looks pretty good as far as size but it's too wide or by mistake it's just a tad too far toward the barrel and hits the barrel when you fire.
LD