What you'll find, in ardent research, is whatever the same cartridge delivers today with blackpowder and blackpowder substitutes (not smokeless loads to replicate old loads) is going to be very similar to what it delivered years ago when it first started. Variability in gunpowder was lining out relatively well by the time we started putting it in metallic cartridges. Accuracy in manufacturing wasn't quite what it is today for mixture consistency and equally for charge weights, but by and large, it's not THAT different than what it is today.
So if you ran 40grn of FFg in a 45colt under a 250grn LRN today vs. BP in 1885, you'd most likely be within a hundred feet per second or so. Swap to a smokeless powder, which can run a lower peak pressure with a sustained burn, you'll get a lot more out of the cartridge.