To really help you out, let us know what muzzleloader you're using, as well as what specific powder and bullet combination. Usually, 100 grains of powder is a good starting point, but it really depends on which powder you're using. Also, let us know what your current grouping is, so we know how much improvement you're looking for.
If you're shooting off a bench, with a scope, most muzzleloaders should easily get you under 3" at 100 yards.
Let us know and I'll be happy to help you get it dialed in before season!