It's confusing because there are several variables. During the Civil War around 15 to 18 grains of fffg were used so that powder charge is a good standard. These military loads were used with conical bullets that were long and pointed and took up a lot of room in the chamber. If you use a round ball you can actually put in more powder, I'd say 20-22 grains is around the maximum.
You need to seal the chambers. One way is the time honored Crisco. Put in the powder, then the ball, then fill the end of the chamber with Crisco.
Another way is to omit the Crisco and use a lubed wad. When this is done, you put in the powder, then the wad, and then the ball. Some folks like this better than the Crisco because it is less messy. No fingers dripping with Crisco.
Finally there are reduced powder charges for target practice. You can use either of the above methods with less powder. Using less powder means when you ram the ball down on top of the powder (you CANNOT have any air betweeen ball and powder) that the ball will be seated deeper in the chamber. Some folks claim better accuracy is obtained when the ball is closer to the front end of the chamber so now there is a filler added. The procedure would be powder, then a filler (like cream of wheat cereal) then ball, then Crisco.
For starters, to keep things simple, I'd go with a 17 grain charge. If 15 or 18 grains yields better accuracy then use that, add the ball and use the Crisco. That's how most percussion revolvers were fired for years.
The only other thing I would add is there is a recent trend to also use a powder measure. Dump the charge into the measure and then the measure into the chamber. The chance of a flask exploding with a percussion revolver is likely very low, no reported events I've ever found, but still a painless and easy thing to do.
