Yeah, that's the question I had too--whether the viewing screen is a dead-center arrangement with respect to the bayonet mount of the lens and the film gate. They could be all over the place for all I know. For instance on a Honeywell Pentax Spotmatic, the screen is 92% FOV and off-center in both directions. Dismal.
I remember from old camera magazine tests that it was very common for cameras to have viewfinder images that were off and even tilted. I used to wonder why they couldn't have made a fixture that would insert in the film gate and the screen opening and align them before the mirror box was tightened.
I do use a digital device in my photographic pursuits when necessary.
When someone rags on me for using film, I use a middle digit, upraised.