Here is an some information about the the spot angle from the Mamiya Forum:
Originally Posted by Paul Sorensen
"The spot measurement angle varies with the lens in use. For example, the 500mm lens has a 1 degree angle, the 150mm is 5 degrees, the 80mm is 8 degrees and the 45mm is 15 degrees. "
I am not sure whether this works linearly, but in the case of the 80mm lens, which has an angle of view of 47 degrees, the spot angle is approximately 1/6 of the angle of view of the lens. In comparison the 150 mm lens has a 26 degree angle of view, so the spot angle is about 1/5 of that, and the 45mm lens has an angle of view of about 76 degrees, so again the spot angle is about 1/5 of that.
I expect that these angle of view measurements are based on a horizontal format. I don't know what the shape of the area of sensitivity is (round, eliptical but horizontal, eliptical but vertical, or ??)
Thinking about all this, I think it might be useful to get some clear plastic, cut it to the size of the viewing screen, and pencil in markings near the centre showing the area that represents about 1/5 the angle of view. Then compare that to the markings on the viewing screen. Finally, find a point source of light, and carefully move the camera, so as to observe where on the field of view the spot meter reacts to the light, and where it doesn't. That would likely give some information about the shape of the area of sensitivity.
A grid screen would probably help too. It also may not be possible to insert the clear plastic between the viewing screen and the prism.
Does anybody else have any thoughts about this?