Our old eyes (or any for that matter) cannot focus on on 3 things at the same time,rear,front,target, The focus has to be on the front sight (not with a shotgun). That is whypeep sights have such an advantage over open sights,you just look through them. For open/peep sights gun fit is very critical especially for a running target.When I was growing up (long time ago) the rule of thumb for gun fit was pick up a rifle and shootas fast you can at a 5 gallon bucket without aiming,If you can hit it then the gun fits. With scopes and treestands and today's type of hunting this isn't very important.
Ken