My 1889 Lefever Arms Co catalog recommends for a 12ga gun of 7-8lbs wt up to 3 drams-1 oz. Then for 12ga 8-9lb guns up to 4 drams-1 oz. You'll have a hard time getting any more than that 3- load into a 2 5/8" shell, with the heavier loads requiring long hulls. Other than muzzle loaders its generally hard to over load a black powder gun, because their shells were built around the correct charge & there's just no room left for extra.
I would not however recommend taking an old gun & loading these charges using 4FG priming powder, that might well lead to some fantastic pressures. In fact I would personally not recommend loading anything heavier than a 3-1 1/8 load in a 12ga with 3FG powder. Anything over this I would switch up to 2F or for the very heaviest ones in the long hulls even up to 1FG.
I do agree, Black Powder is capable of building considerable pressure.
