Stophel,
My Late Ketland Chambers works better bevel down/edge up.
Once I extended the leather out to the edge of the flint, rebound knapping went away.
I am ashamed to admit this, but the last flint went 50 or 60 shots with no knapping. I would look at the edge, and it looked dull as a stump. BUT, it kept on sparking just fine. So, I left it and watched it. The last time I shot I had to replace it.
So, in my limited sample of one, a dull flint, bevel down, still sparks like mad in a Late Ketland. Not sure I'd bet on that with other locks, but there you go.
God Bless, Marc