Before the chart with the USAF targets, gray scales, etc. can be photographed, it should be suspended vertically with the decent precision.  The camera should be placed exactly in the middle of the chart, and there is almost no room for error  - the depth of field for 100 mm lens at F8 at 100 inch distance is just 10 inches.  All printouts have been  glued with the rubbing cement  to the foam board ( let's assume the board is flat.)  At the center I have placed a round mirror around 5 cm in diameter attached with the pushpins for easy removal.  In hindsight, I guess I should have gone to an art store ( say Blicks) and bought a prefabricated canvased frame 24x36" for $20 . Now  I have this contraption hanging on a wire against the wall.  How do you make it vertical? I guess you recognize those screws ;-)   I discovered such a thing as a self-leveling (!) laser levels. I bought the cheapest one for $25 which was almost useless, but the next one for $60 had important feature:  th