John Manton flintlock shotgun would best be loaded with 2F or 1F. It is a general, but not hard rule, that 3F is for small-bore guns, bores smaller than 50 caliber. My largest caliber is a 54, and when I change from my usual load using 2F and substitute an equal amount of 3F, the increase in recoil is noticeable. It was also recommended to me here that reducing velocity tightens shotgun patterns, which may be of interest. 4F, as noted, is for priming.

Great looking gun!