Hodgdons say reduce load by 15% by volume for equiv to BP.
Its a bit unscientific as a comparison, 777 burns faster and breech pressures are slightly higher but still well within spec for use in BP firearms.
I am using 12gr by vol (17gr by wt) in my 44 with a 200gr conical but I have had better in the past with higher load so still have some work to do. Going to increase a bit and hope for a flatter trajectory. The lighter load worked well with a lighter round ball but I was wary of the increased pressure with a heavier bullet.
I think its important that anyone considering trying m/l does not see it as some sort of black art, its not its easy and fun. It is no more technical than reloading cartridges. Just a case of finding the right powder and bullet combination to suit your shooting and your own gun.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane