Try a main charge of 2f and a pinch of 2f in the pan first and see how it goes. If it's slow, you can safely grind a small bit at a time down to a finer grain with a non-sparking mortar and pestle and try again (I wouldn't do more than perhaps a soda bottle lid's worth of powder at a time and wear safety glasses or a face shield just in case) . As above, Pyro doesn't work well as priming and will likely give serious hangfires as a main charge in a flinter. If you want to save some of your real black and shoot some of the Pyro, I'd do as Zonie suggested and load a small amount of 2f downbore first, followed by a charge of Pyro. Since this is a pistol that will likely only need 20-30 grains to shoot well, I'd try 5 grains of 2f, then 15-20 grains of Pyro on top of that and see how it goes. If needed you can adjust the amount of 2f up to Zonie's suggestion of 10 grains and try that (10 grains 2f, followed by 10-15 grains Pyro, for example).