What do you want/need to do with your smoothbore?
Target games, ie- trap, sporting clays, or skeet? Then load FG or FFG and enjoy wonderful patterns. Do your thing and enjoy it! Do you hunt? Well then you'd better think about FFFG or maybe even FFFFG.
For rabbits and squirrels anything will work from 7 1/2 shot to 4 shot (lead) or 7 shot to 3 shot (bismuth) and any reasonable charge of FG, FFG or FFFG.
However if you desire killing shots, not knock out of the air and have to pursue and wring necks, on larger birds such as pheasant, Canada geese, cranes, turkey, and the like, you need to consider a finer powder than FG or even FFG. Geese for example take big shot and high velocity/energy/momentum to cleanly kill. I use 1 3/4 ounce of BB or #2 Bismuth in my 10 guage over top of 100 grains of FFFG or even FFFFG to obtain the velocity/energy/momentum needed to cleanly kill these large birds. FG or FFG only knocks them down. Then I'm running or wading out to wring necks. Screw that. Kill them in the air. My Pedersoli Waterfowl Magnum 10 guage has not blown up yet and has slaughtered........er.... filled my freezer with countless geese now using FFFG or FFFFG. No pursuing or wringing necks.
People like to discount
@Britsmoothy and his experiences with smoothbores and game hunting. Folks should not do that. He tries to offer constructive advice and folks seem to disregard it as BS. Well he has probably killed more game with BP smoothbores than anyone else on this forum. Might want to listen to his advice.