As little as will make it fire. I have one that wont fire with less than 30 grains. The other one works well with 15 grains. That may not sound like much but you better hold on to it with the 30 grain loads.
I totally agree with what PD says.
I've been "bitten" between the thumb and first finger by the hammer on a Derringer, the "bite" was enough to give me a whole new respect for that little guy as it was truly a handful!
I have owned a few over the years, all were .45 cal.
For plinking, I don't recall ever using more 20gr FFFg and that was aplenty.
I've never fired that little Lincoln, but reading PD's advise on "as little as will make it fire" sounds like pretty solid advice to these old eyes.
Uncle Russ...