I was wondering about this as well. I just purchased 2 Aimpoint Pro's and was curious about the mounting position on the AR. Seems everyone mounts them so the end of the Aimpoint stock mount is right at the edge of the delta ring which makes sense obviously but I have noticed that even on AR rifles that have the monolithic type rail (no delta ring) on top (which one of mine does) that people still mount the red dot sight so that the stock mount just matches up with the portion where the rail meets the upper receiver.
Is there a specific reason for this? Is it so you have a return to zero spot? I thought the farther out front on the rail the red dot goes the better overall sight picture you have? Seems almost all the pics that I see even with the T1 sights use that same reference point on the rifle rail though.