Some solid advice here already. However, allow me to add my method to the list. Have used this same method on a variety of pistol cartridges, from .38 Long Colt, to .45 Colt. Including:.44 Mag/ Special. Follows pretty much the same procedure as figuring out a safe powder charge, using Trail Boss. But, was doing it this long before TB came out.
First: Determine seating depth of the bullet you intend to use. Easy to do: Measure from bullet base to crimp groove. Once you have it, mark that measurement on a small, flat ended rod, pencil, etc. I usually just wrap a bit of tape around the end of the rod, then mark it at the right depth, with a pencil, or fine tipped magic marker. Then, mark another spot, about: 1/16"- 1/8", lower down the rod(Nearer the end. I usually mark both). Fill the case with the selected granulation of powder, ie: fffg, to the level of the first mark. Then, add a bit more powder, until it corresponds to the second mark. Dump the powder into the measuring pan of your scale, and note the number of grains. This will be your powder charge, including compression. During the loading process, measure each powder charge individually, just like you would loading smokeless powder. Finally, crimp the bullet firmly into the case. Sounds more complicated, than it really is.
Loading black powder pistol cases isn't an exact science. So, a few tenths of a grain one way or the other, will make little difference in the outcome. Luckily, there is another, easier method, which removes all the tedium of measuring each powder charge individually.
Most folks I know will use a scoop of some kind( cut down casing, Lee scoop, etc), that closely matches the scale reading. Dip a scoopful of powder, level it off, dump into the case. Faster and easier than measuring charges. My preferred method, once the 'master' charge has been determined.
Some folks will use an overpowder wad, some don't. The primary use of the wad is simply to control the charge weight, while still getting sufficient compression. If you want to reduce your powder charge from, say: 30 grains, to 20, a felt overpowder wad can be used to fill in the space. Effective, but time consuming. Generally speaking, I don't bother with this, and simply load the full charge, with the bullet crimped down firmly. Hope some of this helps.