I think Mike has it.
Most pistols in this bore would use 25 -30 grains.
However, (there is always a "however" !)
I have a little old William Bailes, "Queen Anne" pistol with a turn off barrel. As you know, the barrel is unscrewed for loading, and powder put in the breech chamber, a ball sits on top of this in a cup, and the barrel is screwed back on. Ball is oversized, so gets swaged down on the way out.
This pistol is about 22 bore.
I measured the capacity of the powder chamber, and it holds 40 grains, or 1 1/2 drams in old measure.
These pistols were known to be very powerful, and I can attest that it rips a real hole through a burning barrel!
Best,
Richard.