I've been wondering if this would turn up here.
Use an indicator in one position relative to gravity. You can rotate it around a vertical shaft, but not a horizontal one.
In the last century, in the automotive industry, I had a job re-aligning two opposing boring heads. The two heads were mounted on bearing blocks 8" square and 30 inches long, hydraulically moved fore and aft. The part, a brake disk, was mounded in a collet for machining the bearing diameters and seats. Each boring head machined one side. (Inside and outside) Tolerance was Mount the disc between bearings, the disc shouldn't run out more than .0030. Engineering had determined that one of the bearing bores was misaligned, it was my job (over Christmas vacation, at triple time). I wound up building an indicator holder out of 6 inch bar stock, two feet long, to reach from one head to the other. (The collet was removed to allow more easy access to the two boring heads). I screwed an indicator to the end of the 2 foot bar, and had no difficulty aligning the two heads in X, horizontal position. Part of the problem was that to check alignment, it meant tearing down my set up, putting the machine back together and cycling one part. Of course the part had to be carefully marked so I'd know what I was accomplishing. I finally determined that the 6" dia. two foot long bar sagged .002 at the end. I had to put it out of alignment in Y (Vertically) by those .002 to get two bores concentric.
Subsequent to this I made a solidly mounted indicator on a steel bar, not magnetic attachment, and by turning it upside down could illustrate as much as .007 'sag.' I could not couple it in any way to totally eliminate 'sag.'
