https://www.p-max.uk/cgi-bin/black_powder.cgi

This will show the difference of F,1.5F and FF powders.

My 12 bore has a 1-72 twist and shoots a light load 0f 4.5 drams very accurately. [not forsyth]

At 6.5 drams, you are starting to get into some power. FF

I need to find some 1.5F to try.