I began shooting 2F in my 45 (cos that's what I had), and found 40 gr to be good to about 25 yds, then rapidly fell after after that. I guess that's about 30 or 35 gr 3F. I can't imagine using less than that unless you're short on powder, or unless you think that's going to make more of a mess than a smaller load - personally I'd go for the slightly larger load with a slightly better chance of an accurate head shot!
I now use 3F and shoot 50 gr and am in the process of zeroing in at 50 yds, but I think that might be my optimum target load (maybe 45 gr will do).
All this in my humble inexperienced opinion after only a few hundred rounds.