It is generally accepted that 777 is about 15% stronger [more potential energy] it is also faster burning which increases the pressure faster, I personally dont normally use more than 110gr in a rifle rated for 150 of black powder or pyrodex but I have a friend that is using up to 120 so far he has not had a problem.
AS for the differance between FF and FFF it changes with what powder you are talking about with 777 there does not appear to be as much differance as there is in other powders.Lee