CK,
Sight adjustments to change point of impact (elevation or windage) are a proportion between change needed, point of impact, sight radius and distance to target.
Sight change needed divided by sight radius equalls point of impact (high or low / right or left) divided by distance to target. All dimensions are in inches.
Since you have already modified your hammer to some extent and I don't have a M'1860 in front of me, the example that follows assumes a sight radius of 11" and your first results with an unmodified hammer. You can make adjustments to the example after your next firing test and with a good sight radius measurement.
Sight change divided by 11" = 12" divided by 900" (25 yards x 36" per yard).
Sight change = 12" x 11" divided by 900"
Sight change = 0.147"
In this case 0.147" additional height needs to be added to the front sight or removed from the rear sight or a combination of both (some added to the front sight and some removed from the rear sight) as long as the total added and removed = 0.147".
Make sure of the load that you are going to settle on before making the changes as once they are done they are done. Approach the changes on the safe side and tweek the dimensions a little at a time and test fire after each tweek.
Hope that this helps.
HHW