With a properly tuned lock, you should get 80-100 strikes per flint, or more, from any Fuller Flint. As to knapping, If the flint is striking the frizzen at the right angle( 60 degrees) it should self-knapp, and there should never be a reason to be hitting that edge with anything.
To knapp a new flint to your gun's frizzen, lift eh the frizzen up until the edge of the flint, when the cock is released( fired) will just hit the bottom edge of the frizzen face. Hold the frizzen in that position with the side of your thumb. Cock the hammer back to full cock, and pull the trigger. The sharp angle will knapp off a straight edge across the entire width of the frizzen, leaving a NEW sharp edge that will strike the entire surface of the frizzen for each strike thereafter.
When a flint shortens, but still has useable length, use a stick to wedge the flint forward in the jaws so that you can use it further. When you remount the flint this way, you may have to knapp a new edge to marry the flint to the frizzen, again, but this will decrease the existing ( dull ) angle that the flint ground down to, in the process, and give you another new sharp edge to use. I typically get 30-40 strikes from a flint before I have to move it forward.