It's a matter of preference. If you have iron sights you would obviously mount the red dot ahead of the rear sight. The middle of the upper receiver is a good starting point. Go shoot the gun and figure out if it needs to move in or out to make you comfortable shooting with both eyes open. It' all about trial and error to find the sweet spot for you.
When injustice becomes law, rebellion becomes duty.
It will if your optic dies at the wrong time. Check in to the rear MBUS 2 for about $58.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together. You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek