The FFFg gives the grain size with FFFg being finer grained than FFg.
IMO a FFFg powder will work well in your pan but only if it is real black powder.
The most common brands are Goex, Kik, Schuetzen, Swiss and a few others.
There used to be a brand called Elephant which was made in Brazil. Their powder was pretty poor but even so it will work much better than any of the modern substitute black powders like Pyrodex or 777.
While some will say that you should get FFFFg (4Fg) powder for priming the pan, IMO it really isn't worth the trouble.
You can not use 4Fg powder for a main powder charge without possibly damaging your gun because of its fast burning speed.
You asked about how many grains to use in the pan and the answer is that would depend on the size of the pan.
You should use enough powder to cover the entire bottom of the pan but not enough to begin to cover the vent hole.
For some locks that would be about 3 grains.
Speaking of 3 grains, sense a pound of powder weighs 7000 grains, buying 4Fg powder would provide enough to prime your pan 2,333 times.
That is why I feel that for the average shooter who may fire 25-35 times a week at most, buying a specialty powder like 4Fg is a waste of money.
The 3Fg can be used for both the priming and the main charge.