Look at shotgun loads as 'dram equiv'
What is the gun you're shooting?
If it's antique - "
have qualified gunsmith check condition for safety..." before shooting.
3 or 3-1/2 Dram black powder, equal volume shot or weight of payload to suit your need etc.
Separate with greased wad, card over powder.
Select wad and card diameters to suit your hulls. 12ga for plastic (which will be good for at least one load) or 11 or 10 ga for the brass hulls (which can be more trouble than they're worth)
Firm roll or star crimp with good compression.
2f is fine, with your 777 start with 3 dram and see how it patterns. Start with 2f, if patterns are ok with 3 drams go ahead and try the 3f with the same volume.
I've shot black in 16bore shotguns and double rifles for about ten years. The process is this simple, much easier than you could imagine.
FZ