lxdude, I really appreciate your attention to my thread, and anybody else who might answer. I'd like to throw a thought out there for anybody to shoot holes in my logic. So...
If the mirror is off it's 45 degrees, and the front (low end) is drooping too low, then does that not mean that the image will be moved UP on the screen? I have constructed a screen from a Rolleiflex that stands exactly where the film would be ąthe thickness of cheap grade scotch tape, which is about .015mm. Pretty precise, for home-made boob work. So far infinity on the camera screen and the back Rollei screen matches up very well, telling me this 553 ELX is already OK.
But this is a learning experiment. So back to the mirror angle question: If a mirror that droops in the front, moves the image UP on the Hasselblad screen, then the Moon placed dead center on the back Rollei screen, is off-center high on the Hasselblad screen, then is that not the proof that mirror angle is off?