I am about to mount some steel pipe which spans from one wall of a room to the opposite wall. (Here's why: Mount Pull-up bar from wall to wall)
How do I accurately mark two points on facing parallel walls so that the points are directly opposite each other? By directly opposite, I mean that the line joining the points should be horizontal and should also be perpendicular to both walls.
Additionally, I have a hook suspended from my ceiling and it would be nice if I could accurately mark the points so that the line joining them passes directly below this hook in addition to being level and perpendicular to the walls.