Accuracy. Only.
This is something every black powder gun owner/operator should do with each and every gun. That's EACH gun, not just one example of each design.
Bench the gun and shoot 25 yard 3 to 5 shot groups. Use the same POA for every shot regardless of the POI; the issue is group size, not sight adjustment. Use the same size ball (or conical, if you really must) for every shot; in fact, weigh them and use only those that are within 1 grain of each other. Use the same lube, if any, the same filler, if any, the same wad, if any, the same grease, if any, and especially use the same loading technique (that is, the amount of force used to seat the projectile) for every shot. In fact, invest in and use a cylinder loading press. Then repeat, exactly the same but with 5 grains more, another 3 to 5 shot group. Keep shooting groups, increasing by 5 grains each time, until you reach the maximum amount you can cram in and still get the projectile in far enough that the cylinder can move into battery. The smallest group size is the most accurate load. It won't be the biggest load.