It is important that the PJ is centered horizontally on the screen, or the viewers will get different gains on different sides of the room. It is NOT important that the PJ is centered vertically on the screen, as long as the viewers eyes are relatively near the PJ lens. Moving the screen up or down a foot or two will make little difference.
BTW- You won't get a usable 2.8 no matter what you do, because either the PJ would be blocking your view, or you would be blocking its view of the screen. A gain of about 2.4 is the best you can do in any practicable installation (unless you want to wear the PJ as a hat).