A rule of thumb I was given, for 2Fg/3Fg real black powder, is caliber to 2x caliber for rifles. And 1/2 cal. to caliber in pistol. So for your 50 cal. rifle, 50-100gr. Somewhere in that span is the best load with a certain lubed patch, 2Fg or 3Fg, barrel twist, and barrel length.
With FFFg, I would suggest starting @60-65gr of powder by volume. No need, unless you are competing, to weigh your loads. A lot of bother for no reason. Get a 60-120gr. brass measure. That will cover a broad range of calibers.
And NEVER use the FFFFg (4Fg) for a shooting load. Only in the pan for flash, or in a pinch as a small starter when using powder substitutes (Pyrodex, 777, White Hot) in a flintlock.
There are quite a few Gents and maybe Ladies, I don't know, on HPa from the Washington Co. area, and could get you involved with a club. It is a good way to learn, and many will be able to assist.
Good luck, and welcome to the Dark Side! Soon it will be a kit, built by someone else with you watching... then a custom... You have a long and enjoyable journey ahead B2H, relish it!