Well, if you had internet connection you could go to the Hodgdon site and see what the manufacuter lists ...
With the patched ball, Hodgdon lists 100 grains as maximum for 777 in FFG, for 1,988 fps.
With conical projectiles 265 to 300 grs., 100 grs. of 777 in FFG is listed as maximum.
Go with what Hodgdon recommends. It's the one with the ballistics lab that can measure pressure as well as velocity.