Hay Walt: I think you had better ask this question on one of the Black Powder forums, because you don't weigh out black powder the way you weigh out smokeless powder.
I have some minor experience with black powder in a cap and ball revolver but none with black powder cartridges. If you are shooting a cap and ball revolver you may wish to consider the Pyrodex pellets, which are 30 grains in volume, cleaner to shoot, easier to load and deal with and purportedly give good accuracy in the revolvers. The few times I've shot cap and ball revolvers we just filled the chambers with powder, seated the ball on top, spread some grease over the end of the chamber, put the caps on and shot away. Not very precise but that's the way they were doing it.
If you are loading a black powder pistol cartridge you can use the Pyrodex pellets to the same effect, or so I have read. If you use straight black powder you have to measure by volume. You may wish to note that some black powder pistol cartridges identify the amount of powder to use, for example - the 32-20 (grains of black powder), 38-40, 44-40, 45 Colt (45-40). Some of the Cowboy Action shooters I have read about love to be able to reload their cartridges with those pellets as it is easier and gives a much more consistent charge.
Black Powder rifle cartridges apparently take some real skill, or persevearance, to load properly and I would recommend you look at the black powder rifle cartridge forum - there are a number of real experts who post there frequently or who can answer your questions for you.
Hope this helps. Mikey.