That isn't what I meant by missing something. I know the variables involved. I meant it might be a Benskin-esque "plain sight" trap.
This isn't as hard or overly technical as some are suggesting, and by understanding a few basic rules of exposure, it's possible to evaluate the validity of a test method like Schaffer's or WBM. To start with, what are the exposure instructions for the WBM method? It's a simple job of comparing the expected results with the two known exposure points in the above question.
Books don't seem to cover this. The more technical books assume their readers are familiar with the values of the variables and don't bother to show examples. More general photography books usually don't attempt to cover it, so the opportunity of working with actual numbers associated with exposure falls through the cracks. I think this deprives people of a very useful tool for analysis or simply for a better understanding of the process. How can someone think to properly analyze something like the ISO speed standard when they don't have the necessary tools.
Isn't it just 10 times? So instead of .8 it's 8...
So, the speed point would be..
125 = 0.8 / Hm
multiply both sides of equation by Hm... 125 Hm = 0.8
divide both sides of equation by 125... Hm = 0.8 / 125
Hm = 0.0064
Likewise, the metered point would be...
Hg = 0.064
Ten times... Seems extremely arbitrary or lucky to be such a round easy to remember number.
When the rest of the conditions are met, but yes, that is the idea.
I should have labeled the exposure units...