its been years since i've done a projector install, but what i used to do would take a 2 centerline marks from the screen to a position close, but in front of and then another behind the projector. to do this, i would use a piece of string. just like in geometry, if you want to find the mid point of a line, you make 2 intersecting arcs on 1 side of the line, then make 2 more intersecting arcs on the other side of the line, then use a straight edge to connect the arc intersections, or bisect the line... exactly at its midpoint.

the concept is the same here, except you are forced to make all arcs on the same side of the line (screen) and use different lengths of string to give you 2 discreet points.

this method eliminates all other things that could come back and bite you, like the room not being square, the screen not parallel to the wall, the projector housing not being square / perpendicular to your reference, etc...

of course you should confirm your final placement with a test pattern and measure for keystone.

if you got them, those rotating laser levels do the same job, but don't make you look like an amateur with your ball of twine.