I'm not exactly sure what type of Iron sights you're using, but in my case I've got the MOE flip-up sights. When I first mounted my red dot, I put it directly over the ejection port. Looked a little weird to me, so I moved it back (towards the shooter) just enough I could still get my flip-up sights to work at the same time for co-witnessing. I believe the Military trains for your face to pretty much be touching the scope, but I'm not 100% about that one. More or less, it's a matter of preference.