I took some photos of my L&R lock as well as the that of the CVA. The L&R has a noticeably shorter throw, but it also has a stronger main spring. The instructions which came with the L&R said to use a 5/8" flint. For comparison, the wood chip in the jaws measures right at 1" long, but I did have to file a notch so it can set back a little further against the cock screw.
Compare that to the CVA which also is supposed to use a 5/8" flint. The wood chip is 11/16" long, so less than your flint but longer than what is supposed to be in the jaws.
Reliable spark comes from the angle at which the flint strikes the frizzen. If the flint is too long it will hit higher and too straight on. Too short and it will hit too far down on the frizzen which will give a more acute angle to scrape off more spark, but it can also dig in to the frizzen. If the problem is that the priming charge isn't lighting off, you may want to try a 5/8" flint, or cut a hole in the leather to clear the cock screw so you can set the flint back a little. Or if that has been done already, knap a notch in the back of the flint. Conversely, if you want to try longer just move the flint out a bit in the jaws; just make sure to tighten that screw down good.
And lastly, if you PM me your address I can loan you my copy of Eric Bye's book, "Flintlocks - A Practical Guide for their Use and Appreciation." I would want it returned after a reasonable time, not more than 6
years months.
Don't make me come to Texas to find you!!!
~WH~