60-65 grns will get ya started, work loads up or down in 5grn icrements to find the best load for your gun.
Kicks less than a 12 ga shotgun, these guns are heavy,, they "push" more than kick.
The key to a flintlock aka;rocktosser is getting the flint/frizzen to spark good,,others should chime in with help for that.
You won't need any more than 85-90 grains of 3f in that thing,,you'll be pushing recommened max load at 90. You'll find a "most accurate load" before you reach that point.
Good luck,,have fun
p.s. DON'T wipe the frizzen with your thumb,,and watch out for people or open powder containers to your right! fire and burnin stuff WILL shoot out that little hole buy the lock called the "vent",,oh, and pick the vent after loading the powder and before ya prime,,it needs to be open and not plugged with powder.