You will need to experiment a bit. Some guns work best with the pan completely full and level with the top of the pan. Others work with just a couple of grains( by weight). Others require the priming powder be tipped away from the vent , so you use half or less of the pan full of powder. The location of the vent hole in relation to the pan often controls how much flash powder is required for good ignition.
Consider using he same powder you are using in the barrel, particularly when hunting, and shooting in damp weather of any kind. The larger sized granules will fire and burn just as quickly, and they are less likely to absorb moisture from the air. Many shooters routinely use FFFg powder to prime their flintlocks, because it is less likely to draw moisture, and they don't have to reprime their guns so often.
I use 4Fg powder to prime the gun when on a range; but use 2Fg to prime when hunting, because that is the powder that goes down the barrel, and I don't want to haul around two horns. I lost a priming horn once, and took two years to find one close enough in color, size, and shape to match my larger horn to complete the set again.
The priming horn does not go out in the woods anymore. In fact, I usually have pre-measured charges in tubes in my hunting bag, rather than carry my main horn, anymore. I also have a true antique horn that was given to me, and which I had restored, that dates back to the early 1800s. I don't want to risk losing it,either. I use it in the field with my smoothbores, because I have a huge leather game bag that I can put it in so not to lose it. It carries enough powder for hunting birds at a game preserve.
ALWAYS wipe clean the pan, and the underside of the frizzen, both sides of the flint, and around the vent hole in the barrel, so that the residue in those places does not draw moisture. Wipe the face of your frizzen where water can condense if the air temperature is between 32 degrees F. and 40 degrees F.