Not quite Archie...777 pellets are reduced in size compared to regular Pyrodex pellets...They are of equal BP equivalent.
It is only when you use
loose 777 is there a need to cut back 15% as compared to BP or Pyro.
2 or 3 pellets, depends on your gun and hunting situation. Are you pushing the envelope for long distance shots?
There is also the "law of diminishing returns"...more recoil, noise, expense for a marginal gain in FPS. But maybe you need it for 300 yard shots.
I will leave it up to the resident expert "SheepHunter" to throw in the data (he is very knowledgeable in this area)
What ML are you shooting anyways?