I mount mine as far back as I can, right next to my rear BUIS. I do this because it allows a greater FOV through the glass.

Think of it like looking through a toilet paper tube. Put it right up to your eye and how much can you see through the tube vs holding your arm all the way out and looking through it?

Also, when mounted farther forward on the rail, you are reducing the total angle you can see the dot from, effectively canceling the benefit of red dots being parralax free.