Welcome to the forum.
If I can give some thoughts on heavy powder loads in a .45 caliber rifle?
The Lyman "BLACK POWDER HANDBOOK & LOADING MANUAL" shows a lot of various powder loads with different projectiles.
Under the roundball loads using a .440 diameter patched ball they got the following using GOEX 3Fg powder:
70 grains = 1777 fps mv with 11,900 psi breech pressure.
80 grains = 1856 fps mv with 13,300 psi breech pressure.
90 grains = 1970 fos mv with 15,100 psi breech pressure.
100 grains = 2084 fps mv with 16,900 psi breech pressure.
I must mention that not only are some of these breech pressures very high (for a black powder firearm) but the steel your barrel is made from is just low carbon steel. It has very low tensile and yield strengths and cannot be heat treated to improve it.
The strength is nothing like even a small modern CF cartridge pistol which all use heat treated steels of much higher strengths.
The thing you may also find interesting is the ball velocity at 100 yards.
With the 70 grain load the 1777 mv dropped to 1012 fps @ 100 yards.
The 80 grain loads 1856 fps mv dropped to 1037 fps @ 100 yards.
The 90 grain loads 1970 fps mv dropped to 1076 fps @ 100 yards.
The 100 grain loads 2084 fps mv dropped to 1122 fps.
Clearly, due to the poor ballistics of the roundball, all of that extra velocity at the muzzle rapidly vanishes with the final result at 100 yards being only 110 fps between the 70 grain load and the 100 grain powder load.
While I can understand the desire to have a trajectory similar to a modern high velocity rifle it just isn't possible.
Rather than trying to get modern performances out of a 150 year old design I suggest that you accept the limitations of the old guns and take on the challenge of getting exceptional accuracy instead.
Once you find the most accurate load, learn what it is doing at various ranges. (Many zero their sights at 70 yards because if done, at 50 yards you will be about 1 inch high and at 100 yards you will be 4.5 inches low.)
Using your knowledge of the trajectory you will be prepared for any hunt out to (my recommended) range with this caliber of 90 yards.
The gun with a powder/ball/patch/lube combination it really likes is a very capable arm.
The weakest part of obtaining superior accuracy is the traditional iron sights and many have found that even with these limitations, shooting a 2 inch group at 100 yards can be done.
Most of all, have fun.
This sport can be great fun and conquering its challenges have convinced many shooters and hunters to give up on modern CF firearms and devote all of their shooting to enjoying these "old fashioned guns".