The ethical distance you put on your muzzleloader shots, IMO should be in direct correlation to the caliber of the rifle, the projectile being used, the powder charge, the events you face in the shot, and the proficiency of the shooter.
For instance... we have a few shooters on the forum that practice out to distances of 200+ yards. And I mean they shoot a lot, and hit their targets. I think if you ask them they would be the first one to tell you that while they could make the shot at extreme distances, have faith in the rifle and load, in reality hope they would not have to. While this 200+ might be their ethical distance, they would also tell you that if the wind conditions, topography of the area, even the animal itself presents any distractions to their comfort level, their ethical distance has just shortened and the shot would not be made.
What bothers me is what I call the slob hunter. We all know one. They are the ... oh look a deer out there at 200 yards, and while they lack the skills to make that shot, blast away. They have no ethical distance. I stopped hunting with a person for that very reason. It was either stop hunting with him or stomp him into the ground. The shots he was taking were unethical, and he knew it.
I once made a shot at a whitetail deer at just an obscene distance. Estimated at over 400+ yards. This was done with a center fire rifle. The caliber was made for this (7mm mag). I had been practicing for elk hunting out west and at the request of my future guide had spent countless hours and cartridges practicing out to even 500 yards (but had no intention of shooting that far). Friends and I came upon a herd of doe, we had to fill a doe tag that year, and the deer were in the middle of a huge picked corn field. They all stood in that field looking at us in the woods on the edge of the field. My friends there were all unsure of what to do. I got my rest set up, and settled into the shot. I decided to aim for the head of the deer. Why... if I missed I would miss I figured. Not judging the distance correct, I fired and hit her lower then I thought. It was to the front of the throat under the jaw, broke her neck, severed her jugular vein, I mean she dropped. So after the shot was made, my friends were all back slapping, bragging me up, etc. I felt terrible after taking the shot. I knew I had made an unethical shot. I really had no right to attempt that. Even though I had practiced.
With a muzzleloader I limit my scoped shots to 125 yards. And have decided after this year to limit my open sight shots to 75 yards. That might sound far to close to some of you. But to me, I know without a doubt, I can make the shot at that distance if I have to.