As a general rule, the Red Dot can be placed anywhere on the flat top just above the receiver as far forward as the forward guard. Why, well because shooting the Red Dot requires both eyes open (you can even have a lense cap closed on the front of the Red Dot (so you can't see thru the RD at all). You brain will place that red dot to your other eye and you will only see a RED DOT on your target.

Now with that said, most place it right above the ejection port. Some that use a magnifier need more room behind the RD. So that is the basic placement. Is that the best location. Well LEO and Military places them there and that kind of says it all.

Good luck, don't forget about backup sights, Iron Sights, and co-witnessing, just ask on this and you will be helped by many wonderful shooters... But that's another story... Keepem in the Xring.