The shorter end of the square goes along the top of the screen. Right at the center mark you extend the long end to touch the ceiling and make a couple of tiny pencil marks where that spot is. You'll most likely need a level to get the center mark to the ceiling. Now lay the square or other straightedge from that mark to the extended one you made and make a few more little marks along that line. Put the back edge of your laser against the ceiling and make sure your line follows the marks. I've had a screen off kilter enough that the rear measurement to the center of the lens has been off by almost 4 inches! (I probably should have shimmed it out but I thought that might look weird).