Hi deerheaven,
It depends a lot on what range you expect to do most of your shooting at. If you expect to do most of your shooting at about 100 yards, then you'll want your point blank range (meaning the range where the bullet hits exactly where your sights are as opposed to above or below your point) set at 100 yards. Now a lot of this is going to depend upon the loads that you work up, but basically if your getting muzzle velocities around 1800-2000 fps, this will work. With that velocity, if you're sighted in at 100 yds, the bullet will hit dead on at 100 all the way through 115 yds. After that you have to compensate for drop.
To get this to hit point blank at 100 yards, you'll actually be shooting up a little bit. So, at 50 yards your shot will be about 2" high. So, at 100 to 115 yards the bullet will hit right where your sights are set, and everything in between will be within 2" of your sight line. That will pretty well take care of most deerhunting with a muzzleloader and open sights.
To get sighted in, you need to put it on paper first and at 12 yards the bullet hasn't gone above your sight plane yet, so that's a good starting point. Once you have it on paper at 12 yards, move it out to 50 yards and make sure your still close. You should be a little high at this point so whatever else you do, DON"T file down your front sight blade yet. Make left and right adjustments and adjust the rear sight if it's shooting a lot more than 2" high. Finally move out to the 100-yard mark and get it as close as you can to dead-on.
If you sight the gun in for 50 yards, you should hit dead-on right out to 50 yards. But then at 100 yards it'll drop about 3 3/4" to 4", which isn't bad, but at 125 yards it will drop 8-8 1/2", which sucks. If most of your shooting is at 50 yards or less this works fine, but if it's out at 100 yds, you should sight in for 100 yards.
Hope this helps.