I used to us FF in my flintlock (.54) years ago and used 100grs...When I switched to FFF I worked up a load again and about 80grs was the sweet spot...By moving to FFF you will have a cleaner burn and less residue in the barrel...It's hard to guess how much your gun would prefer without shooting...Assuming it's a .45-.50 or .54 anywhere from 70-90grs is plenty for deer...