I have not used a grey card. If I understand correctly, the issue in development is mainly with contrast correct? This is essentially what I think Barnbaum is recommending in his book in having two backs. If I shoot 10 shots of snow with dark tree line and 10 shots of dense fog on the same roll, in one way or another, either the contrasty shots (trees and snow) or the un-contrasty shots (fog) will likely suffer to some extent in development.
Am I on the right track?