There is no "proper" amount of prime, if the gun go off, that is how much you want to use. You can get a small sized horn with a plug vs. a plunger, or, just pour out of the main horn into the pan. Just pour a small amount in the pan and as said, about a third, to maybe half full will probably be a good amount. May take some practice for you to find just the right amount that you are comfortable with, too much in the pan and you may singe your eyebrows
and get slower ignition, and too little and you will not get reliable ignition. Studies have shown that banked against the touch hole gives the fastest ignition, however again, you will have to experiment to see what works best for you.
As far as using 2f in the pan, it depends on your gun. I know people with besses use 2f in the pan and their guns always go off. The British military (as well as many others) used paper cartridges for loading, they tore off the end, primed, then poured the rest down the barrel to load. If you are using 2f, there is no
need to use smaller grain for prime, again your gun will tell you by how fast and reliably it goes off.