Quote Originally Posted by Bill Burk View Post
It's NOT in the middle. It is a full stop up from the middle. But wherever it is - it is there for coincidental, arbitrary or scientifically chosen arbitrary reasons.

The scale on the Weston Master III progresses

Which corresponds to

Zone System

There are four stops beneath V and three above.
Four and 1/3 stops below metered exposure point actually. Bill, go back and re-read where I said it is about the middle in the camera image. For my money, it is a good possibility that this is where the calibration point decision originated. 12% is a stop off from the mid-point of a full range subject, but dead center of the camera image of the full range subject. I'm not saying this was the only part of the decision, especially in the early days, but I don't think it is a coincidence.