By memory (suspect!) the guys that were involved in hind legs shooting at 200 yards, used a duplex load not exceeding 10 percent of the full load, of smokeless powder under a main charge of BP. Pope made a powder measure that actually measure out the two different powders, for reloading on the firing line.
This was the guys like Pope and his peers, shooting Scheutzen (sp?), usually with cartridges loaded with powder on the firing line, and bullets that were seated over the powder charge by loading them from the muzzle, or breech seating them (pushing them in to the rifling at the chamber end of the barrel). Mostly, these guys were shooting 32-40, and similar.
By my reading, the use of 10 percent smokeless powder, had the advantage that the usual BP fouling was not an issue.
But, honestly, I'd be wary of dicking around with the BP subs, as they are still not in the same pressure curves as BP itself is.