Bell-housing alignment help

My math brain is kicking in on whether the dial indicator has to be as close to perpendicular as possible. It if wasn't perpendicular, say 20 degrees off, a five thousands reading on the dial would actually be more, say seven thousands, just making up a number. However, this error would be replicated 360 degrees and cancel out. Too lazy to put the numbers on paper to prove or disprove my thoughts....