Hasselblad 500C Viewfinder + Calibration

[sonora]

I wonder if a rear protective body cover could be used for that, too?

It depends... I just visually compared my metal rear cover with the film plane indicator on an A12 back, and although they look close, I don't know if I would I would trust it to make critical adjustments to a focusing screen. I'd rather just use a trusted lens at the infinity stop and focus on something very far away (like the moon), and just adjust the focusing screen until it's as sharp as I can get it. Then verify with a couple test shots at various distances, maybe taking a digital photo of the image in the focusing screen to compare.

There are also a few threads already on the topic of adjusting/replacing 500C screens, so worth looking at those as well... this may be a good resource, if you haven't already seen it: https://www.photrio.com/forum/threa...ng-a12-film-back-internal-light-seals.217346/
It also links other threads on the topic.