With a 54, you really need to up the powder charge and switch to the holy black. Order some through a NMLRA charter club, a reenactor group, or find a N-SSA member to help you.
As for hitting high, it's a bit of a paradox, but light loads on a larger caliber with a heavy bullet do tend to shoot high and that's due to the bullet still being in the barrel during the recoil impulse and that gets worse as the barrel length increases. A couple other suggestions- when shooting from the bench, do not rest the forearm on the sandbag as you would with a modern rifle. Grip the forearm where you would for offhand shooting and rest the back of that hand on the sandbag. I can say for a fact that tends to tame lighter loads on large caliber muzzleloaders shooting high. One more thing, use a center hold on all targets when doing load development with a muzzleloader. Otherwise you're counting on the bullet hitting where you really aren't aiming. It's hard to tell if your front post is exactly at the tangent point of the bottom of a circle.
Here's a pic of a group I shot with a 54 Kibler Colonial. Owner was shooting 5in higher and about a 6in group at 50yd. We changed nothing on the load. I shot the rifle using the exact bench technique I just described so here's photographic evidence it works. The owner was shooting all over the upper half of the original target.
View attachment 134593