I agree I don't know how it could be measured. I'm just asking the question from a theoretical perspective.
Suppose one plots a ZS-style Net D/Log H curve with the usual goals of finding an EI speed point net D of 0.1 and Zone VIII net D of say 1.20. If the measured zero-exposure fb+fog density is 0.30, and one measures a Zone VIII gross D of 1.50, the assumption is that Zone VIII net D is 1.50-0.30 = 1.20. But what if at a gross D of 1.50, there is actually 0.35 fb+fog density? In other words, what if there is a >0 slope to the fog density "curve"?