I don't load for .32-20, but I do use a .32-20 case as my 21 grain measure!
The accepted way to load BP, at least for rifles, is to use a drop tube. I use a 24" piece of copper pipe mounted on a stand knocked together from a coupla pieces of 2 by 4. Some use an aluminum arrow shaft. A long metal supply tube from the plumbing display at your local "Lumbarama" store looks promising. With a 1/2" pipe, I slip a reducing fitting on the bottom and stick a funnel in the top.
Measure by volume, but check-weigh on your scale to make sure you are on track. That 20 grains sounds about right, dropped, and about 1/16th" to 1/8th" of further compression when seating the bullet. Some use a "compression die" but if used too enthusiastically the grains at the top of the load get crushed. Some also recommend a card wad over the powder, but unless you are trying to knock over a sillouette at 500 to 1,000 yards I think it is only an option.
Cruise the boards as there is a lot of advice on loading BP available. Lyman puts out a very good BP loading manual.
Hint; A .40 S&W case drops 18.5 gr. poured in from a can spout or dipped, levelled in both cases. (you better weigh it yourself - Boy!)