You bring up a good point on leading a deer. They are not slow, deer run about as fast as a duck coming into decoys. The buck I mentioned in one of my scenarios, he was right about 40 yards, perfectly broadside (crossing shot), running full speed. I can't recall perfectly where I had been aiming, but I'm fairly sure I had the sight blade just a touch in front of the body. The ball, which was a .520" pure lead ball leaving the rifle at maybe 1400 fps, impacted about a hands width behind what I call the armpit of the deer. This centered both lungs up and down, and was about 2/3rd to the back of them. Overall, the impact was 12" to 16" behind where the sight was. With a modern super high velocity rifle, lead might only be a couple inches, or otherwise un noticeable. If you put the bead right where you want to hit with any muzzleloader or handgun, you are going to gut shoot them. Of course how fast they are going plays a part, and distance. You could put the bead right on the shoulder if they are only trotting along at 15 yards. That's all part of practice.
For the most part, I shoot a shotgun using the sustained lead method. When shooting ducks I focus on the head, and let my brain do the aiming. When shooting geese, I focus on the white patch on the cheek. With deer, I've been focusing on the center of the shoulder meat. Not the shoulder itself, but where all the muscle is. I've heard plenty of times with buckshot a good spot is where the neck meets the body, because it does not move much when the deer is running. That, and you get half your shot in the head and neck of a deer, which provides dramatic instant stops.