I use both English black and French amber flints in my Kibler SMR, and install them bevel up/flat side down. The closer you can mount them to the frizzen and still get it to close on the pan, the better. This will ensure that the flint strikes as high on the frizzen as possible, giving the longest scrape and hottest sparks. Like Mike, as the flit wears and is sharpened by knapping, I move the flint forward and place a twig behind the flint to help move it forward. I sometimes get over a hundred shots out of a flint, sometimes only twenty...depends on the integrity of the stone.
When you knap your flint, remember that there is likely only a very small nub that is dull and is pushing the frizzen open without cutting a shower of sparks. You don't have to knap the whole width of the stone, only the part that is causing the misfire. So examine your flint carefully, determine where the issue is and knap that away. Don't wipe off the flint with your thumb after knapping, unless you have a bandaid in your bag.