If you switch to loose 777 powder and want to shoot the equivalent of your '2 pellet 100grns' , you'll have to reduce the loose 777 when you measure it out by 15%(or 85grns).
In other words loose 777 powder is 15% more potent then most other powders soooo... 2 50grn. pellets of 777 is equal to 85grns. 777 loose powder.
Ah, no, I don't think so.
Two 50 grain pellets of 777 is 100 grains of 777, period, loose or otherwise.
85 grains/volume of 777 is, approximately, equal to 100 grains/volume of real black powder or Pyrodex.
100 grains/volume of 777, loose or in pellet form (two 50 grain pellets), is, approximately, equal to 115 grains of real black powder or Pyrodex.