I usually use about 10% less with FFFG than FFG. I think I get better accuracy with the FFG though. So i would try 80 grains.
80 seems to be a peak of energy versus powder volume in a lot of guns. Personally for my area and whitetails, 80 is a good choice. I would sooner go to 75 than 90 on the next trial.
I think another consideration is patch thickness. With a 530 ball my four flintllocks like a patch thicker than 0.015. I use prelubed patches from oxyoke. Thinner patches don't fill/clean the deep rifling and lead to loss of accuracy in my opinion.
I also damp swab between every round at the range. Damp not wet, just enough to make the carbon loose.
So I guess what I am getting at is the best guess of accuracy variables. I would say patches and cleaning are more important than powder. And I would expect a 10% reduction in volume from FFG to FFFG to be close. And like all firearms, a good barrel is more forgiving to variables.
