I assume that since you are on this forum, you are shooting a traditional muzzleloading rifle and not one of those modern scoped abominations. Those devices are not discussed here.
You didn't say if you were planning to shoot at an animal at distances greater than 100 yards or if you were just punching paper but for hunting purposes when an animal is involved as the target, a traditional muzzleloading rifle is a 100 yard weapon. The reason is quite simple, a round ball, because of its shape, looses momentum very quickly after 100 yards regardless of the amount of powder you put in the bore. As it looses momentum it looses energy and as it looses energy, it looses killing power. All taken together, after about 100 yards, the probability of wounding rather than killing an animal greatly increases.
:nono: Any ethical hunter with a traditional muzzleloader will limit his shots at animals to 100 yards or less.
On the other hand, if you are shooting paper and no animal's life is at stake, the sky is the limit at which you can attempt to hit a target. For those purposes, just use a good round ball ballistics calculator along with a chronograph to find out where your ball will be at these long ranges and have fun. Attempting to hit a paper or metallic target at long ranges with a traditional muzzleloader can be a lot of fun. Here is a round ball ballistics calculator:
http://www.ctmuzzleloaders.com/ctml_experiments/rbballistics/rbballistics.html If you don't want to download it into your computor, you don't have to. Just scroll down to the bottom where it says "CLICK HERE to open a web application without having to download anything!"