The powder loads for a pistol should be much less than the powder load for a rifle.
Black powder, or any of the synthetic black powders burns rather slowly and the patched ball would be long gone from the short 9 1/2" barrel by the time a large quantity of powder had burned.
Assuming your rifle loads are in the 60-90 grain size your pistol loads should be in the 25-40 grain area.
Also, because of the reduced powder load most single shot pistols use you can get by with using a thinner patch.
The use of a thinner patch will make starting the patched ball into the muzzle much easier which is a good thing because most pistols are rather hard to get a good grip on them while starting the ball.