Gunflints length is much more important than width. A flint narrower than the frizzen is no problem. A flint wider than the frizzen is no problem. Just mount it toward the outside. It will self-knap. The rule of having the flint just barely away from the frizzen at half cock can sometimes work, but anyone can see that the same flint mounted bevel up versus bevel down will give very different results using this method.
So, the flint must not be so long it keeps the frizzen from closing. But after that, learning whether your lock works best with the flint bevel up versus down, and how high it should strike on the frizzen to both give good sparks and flip the frizzen over reliably, is a trial and error process. Chances are, if you are relying on " almost touching the frizzen", you're not getting best performance.