We have some here using that rifle at long range matches. 20-1 lead /tin is what they use. I don't know of any who size their bullets. per se, but SPG is used a lot. One has a PP bullet he uses. These guns are very accurate, and I think you will have better luck with the softer alloy.
As for hunting, I prefer a round ball of decent size. I know that bullet shape you have, and am not crazy about it for hunting. If you are going to hunt with it, I'd file a nice .30 meplat on the nose.
In the woods here, I lost 2 deer in the swamps after shooting them with my 40-70 Sharps. The 45 cal wasn't much better with my long range bullets. [ unless I hit bone ! ] My 50-70 Sharps carbine was my best game gun , until I went to a .54 round ball flintlock. Since shooting/hunting with round balls, .50 to .75 , I have not lost an animal. That speaks volumes. My moose gun is now a .62 cal Edward Marshal flintlock.
Anyway, good luck with your shooting. First shot accuracy should pick up with the softer bullet .