The advise you have received is all good. The Key here is the words the Co stated is Volume for Volume, not Weight for Weight. If it were Wt for Wt you would actually have to use more of the Pyrodex, not less. A 4 Dram charge of Black weighs about 109 grains, with the 70% density a 4 Dram "Volume" of Pyrodex is going to weigh about 76 grains. Industry standard for a volume measure for Black is .115 Cubic inches per Dram, thus a .460 CuIn measure would be correct for the 4 dram load. With those figures you can if needed make your own.
Pyrodex leaves less fouling in the bore than Black, but do not be deceived into not cleaning effectively. It is actually more corrosive than black but the same cleaning technic will take care of it.
