I use an 8' aluminum straight edge - one designed to be used as a guide for using a Skill saw when cutting plywood. I have a helper hold it centered across the face of the projector frame with the level ends extending equally to the left and right sides. I take a tape measure and measure the distance from the tips of the straight edge to the left and right center line edges of the screen. The added length of the straight edge exagerates any squareness error. I also use a laser pointer shooting down the centerline of the projector frame to ensure the projector is "aimed" at the center of the screen. When both of these are satisfactory, I am as close to straight and square as is reasonable to get.
Good question. I had an unfinished (e.g. unpainted) room to work with, so marked lines on the ceiling between the pj and the screen, then I used a framing square to draw a perpendicular line for the screen. I never measured the distance from the front edges of the pj to the outer corners of the screen, though this would have confirmed squareness.
You can also take two pieces of masons twine and tie them from the front corner of the projector to the opposite corner of the screen. This works best when the projector is on a rolling cart so you can pull it back until both pieces are tight.
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