Thanks for posting.

The very first Army issued SOPMOD Trijicon RX01NSN manual put it all the way forward on the rail next to the FSB.

Forward end of the upper receiver is the best compromise in my personal experience for proper eye relief, target acquisition.

There is no one “correct” position.  What is right for each user is going to vary depending on user preference balancing ability to pick up the dot quickly against peripheral vision around the optic and field of view within the optic.

That said, I prefer mounting a red dot as far forward on the upper receiver (not out on the rail) as the receiver permits while keeping all of the mount on the receiver.  Not on the rail, not bridging the rail. That is the best compromise for me.

I prefer closer to my eyeball which, for whatever reason, gives me quicker target acquisition. Even closer, I have no issue with field of view (2-eye) regarding movement.

I prefer mounting as far back as possible, very close to the eye. This gives you a larger field if view through the glass, and causes the rim to basically disappear instead of obscuring part of your view as it does if both eyes can see it.