I've used 2Fg in .303 British loads before. Used both 2Fg and 3Fg in .45-70 in a modern rifle; straight BP and duplex loads.
For amount it's as much as you can get into the case. Back in they day they either used special compression dies or precompressed powder charges inserted into the cases before the neck was formed (that's what they did for .303B). If you are just lightly compressing, you wont be able to get more in there than it was designed for. I've managed to get 65gr of 2Fg into a .45-70 case with heavy compression that bulged the case slightly. I don't have the equipment to get 70gr into there like the cartridge name denotes.
A .303B case likewise also used 70gr of black powder (I'm not sure which granulation) but I wasn't able to get more than 55gr (2Fg) in because I couldn't effectively compress the powder heavily in a necked case. Without any compression I think it held around 45gr and I ended up pouring the charge in steps and compressing with a dowel between steps. This allowed a little more powder than pouring all the powder at once (even through a drop tube).
Also note that when I refer to "black powder" I am talking about the combination of sulphur, salt peter, and charcoal (I use Goex). Nearly everyone I talk to locally about black powder refers to substitutes like Pyrodex, Yukon black gold, Blackhorn 209, and other substitutes as "black powder". When I ask if they use "real black powder" they enthusiastically say yes, usually followed with an explanation of doing it like in the old days, and how difficult it is to find "real black powder" now a days, and then pull out their bottle of substitute with a smile on their face.