Increasing the powder charge may have the opposite effect to what you think. As well, a change in powder charge may also give a windage change. Trust me on this. Heavier powder charges give higher velocity and the bullet is out the barrel quicker, therefore the gun hasn't risen as high in recoil, therefore ithe bullet prints lower on the target. This is a common phnenominum - may be the same in your .58 - was in my .69 to about 100gr. 2F with 480gr. round ball. After that, it printed higher with heavier loads.
: You must be flexible enough to try something different than what you were SET up with - or put up with where it shoots now. Shooting Black Powder is a matter of experimentation. You must experiment with different bullets, different size bullets, different powders, different powder charges, different lubricants. different caps, EVERYTHING different makes a difference in HOW WELL and WHERE it shoots. You would think lighter conicals would print higher as well - not necessarily so, as they generally print lower on the target from the musket, same as they do from a handgun & modern rifle for that matter. It has to do with barrel time vs. barrel rise from recoil. Some guns, due to stock shape or the way they are held, print the opposite - you have to willing to experiment.