Steve says it well . . .
I have always used 2F on .50 or larger and 3F on anything smaller -
BUT
when I have been short of one or the other, I have just used what I had and adjusted the load. AND . . . in over fifty years of shooting, I have never felt the need or used some of the heavy loads that some folks seem to feel they need to use. But I won't add anymore on that to avoid any confusion.
In my C & B revolves, I PREFER 3F but when short of it, have used 2F. The same on my rifles and smoothbores - percussion or flint - depending on the caliber.
$F (FFFFG)? Keep it for priming flintlocks only.
The only thing I ever used 1F (FG) for in all the years I've been shooting were small (i.e. 1" bore) cannons and a 4 gauge original percussion "market gun" that I had in my collection. A few shots out of that 4 gauge was enough to last me a lifetime.
OP - if you are new to BP, do yourself a big favor and pick yourself up a good Black Powder ML Handbook or Manual. Read it and study it as it will provide you with all kinds of information.
I learned to shoot BP from an old gunsmith that was almost ninety - and that was more that fifty years ago. Reproductions were just starting to show up but usually, originals were utilized. That's what I learned on. The best advice that old gunsmith gave me as far as powder charges was - "use enough to get the job don but not so much as to waste powder". Pretty sage advice. Ideally, your charge should be so that when that projectile leaves the muzzle, all o the powder is consumed. If not, it will fly out the muzzle and be wasted. BP is no different than reloading for smokeless cartridges. Start low and work you way up until you fine the "sweet spot" of your firearm. Even with the same make/model of a BP gun, each one can shoot differently. So work with your individual firearm and determine what it shoots best with.
Be safe and have fun!