I own the 45mm f2.8 and 55mm f2.8. Both are excellent.
Not sure what to say about fine-tuning exposure. A Mamiya 645's exposure settings are quite normal for the era in only offering whole stop aperture and shutter adjustments. Generations of photographers shot touchier slide films than we have now this way and it was fine. Even with slide film, exposure is more art than exact science.
With negative film.. you can't really look at the negative and say, "that's just a *little* too dark/light..." because the neg is not a final image. There's a lot of information in a negative you can't see and which can only be brought out by tuning in the print or scan stage. Overexposing a modern negative film by 1/3 stop is nothing. You can overexpose some negative films by 3 stops or more and still get usable images. When in doubt, lean on overexposure with your neg.