When there is an issue with the focussing accuracy of a 500 C/M, and the focussing screen isn't wrongly installed and the optics are good, it is mostly due to a miss angled mirror.
The mirror is held down by a little hook which is released by the second shutter release mechanism.
This little hook is, to my humble opinion, somewhat 'insipidly' built and gets out of adjustment by use and wear.
After faithful service for more than 20 years, Will van Manen (from Holland) had to calibrate the mirror angle of mine. Will, BTW, does a very good job in CLAting Hasselblad cameras.
Don't try to do this your self, tools like a micrometrical table are needed to do this properly!
If your camera body is a 500 C/M, not just a 500 C, than install one of those (Minolta designed) Acute Matte-D focussing screens, which is easily done.
All-tough not cheap, these are real marvels and worth there money (Will sells them at a rather fair price).
Be happy with your Hasselblad.