I have this issue but with a C33 I recently aquired. I have been sorting through any info I can find about this on the internet. One post mentioned using a ground glass on the film plane and focusing on that and then check the view finder to see if they are the same.
Dave, would you be able to give me a piece of ground glass that would fit within the film plane on my mamiya? If I gave you the dimensions?
I would assume the easiest thing to do is focus the taking lens on the ground glass, then adjust the focusing screen if they are not the same. Does anyone know what the up and down movements of a focusing screen does.
For example. If the film plane is focused and the focusing screen is out of focus do I move the screen down or up? Or is this one of those "you will only know if you play with it" things !!!