It all depends on the eyes of the shooter, the size of their head and how they originally learned to shoot peep sights.
When firing for match-competition type accuracy, I place three fingers from my left hand between the charging handle and my cheeck. It gives me an exact measurement every time and works well for my eyes. For night firing and close rapid fire I move my nose right up and keep both eyes open. (unless I am using an aim-pont then I just keep both eyes open and hope the battery does not die.)
When I met my lovely bride, many years ago, she was shooting her M-16A1 with her eye super close to the rear apeture. It looked weird, but she could shoot just fine. I was going over to give her a hard time about it when some other GI decided to use his rifle to make an inappropriate gesture towards the back end of her BDUs. She took offense and butt-stroked him so hard his helmet went flying across the range. That really got my attention...
Now 5 kids and many years later she still shoots that way....But she has not butt-stroked anyone for several years now.