Good shooting. That is a great looking pistol. I had one in the 80's traded for and shot it some in primitive matches. It always did well for me. Those are good groups . You might get a little better group with 3F. I did in all of my pistols but you just have to try. You are right at the sweet spot I had with my pistol with the powder charge and ball/patch size. I looked at my pistol shooting notes on my black powder rigs and I ended up shooting 35 grn. of 3F and same ball/patch combo. Something I learned in the 80's shooting Bullseye matches from my Shooting coach, is on the position of that group, if you put a little more of the pad of your index/trigger finger most of the time it will bring your groups to the right. Don't know if you are placing just the tip of your finger on the trigger or not but when I first started that is one of the first lessons he gave me. It has something to do with how the pressure on the trigger using the tip of your finger pushing just slightly away from your grip. Moving your finger over the trigger more will bring it back ever so slightly. I have to remind myself sometimes to do this, especially on my Ruger Old Army with the trigger that has been modified with serration on it parallel with the trigger like some old Smith & Wessons. Also on most of the Glocks and other pistols with the safety in the trigger. I thing you are started down the right path.. Good luck. I had a CVA Mountain pistol I built from a kit in .50 that I used once on a deer hunt and took a doe with it from a ground blind. My Flinter and I had been setting in the blind for hours in a fog like condition and when I finally pulled the trigger I had a bad hang fire and shot over the deer. She just jumped and spun but didn't know where the noise came from and it gave me time to take the pistol off of my lap, and using a shooting stick I put one right where I needed at about 20 yards.
Good luck on your shooting
Mike