There's different options for sighting it in.
For starters, 25 yards probably isn't the best distance for sighting it in, but 50 yards would be better.
Also, muzzle loaders often have more than one sweet spot. Rifles often have both an accurate target load and a hunting load for higher velocity shooting. The 55 grain powder charge is closer to the target load which could be adequate for sighting in at 50 yards.
If you shot several different loads at 50 yards loaded with 55 through 85 grains of powder, then you could see if there was much of any change in impact at 50 yards between the loads. From that point it's easy enough to deal with firing at longer distances by using Kentucky windage when aiming.
To raise the point of impact, folks usually simply file the front sight down just a very little bit at a time and closely observe the impact of the loading as it "walks up" the target.
Of course fixed sights will never be perfect for every load at every distance. But generally if a gun is sighted in for one inch high at 50 yards, then it may be about 5 inches low or so at 100 yards.
However that doesn't mean that the windage will be the same at every distance either.
Installing either a new adjustable rear sight or both a matched front and rear sight set is a personal choice.
If only installing a new rear sight, there's a chance that the front sight will not work with it properly due to it not being the correct or optimum height.
That's what may have happened when the rear buckhorn sight was installed.
It's a desirable sight for hunting and faster target aquisition, but it needs to match up with the height of the front sight for it to sight in properly.
So it's important to sight in with an accurate load that groups well at 50 yards.
Then when shooting at 75 or 100 yards, the powder charge can be increased just enough to maintain good accuracy and a similar flat trajectory for the longer distance.
Sometimes one of the sights will need to be drifted to adjust the windage, as well as making the height adjustment of the front sight for elevation.
Make sure that you are comfortable enough with the sight picture provided by the buckhorn rear sight to be consistent with it. The same advice applies to the powder charge that will be used for final sighting in at 50 yards.
A moderate powder charge often works best for sighting in at 50 yards since heavy hunting charges can sometimes be erratic, and the point of impact doesn't change all that much with heavier hunting loads. All of the loads should be shooting relatively flat at 50 yards anyway.