Hi,
I have looked at the graph you mentioned (think it was the one). It basically shows reaction of film(darkening) when exposed to light. Exposure is expressed in T-stops not in logH as usual and 18% gray card normal exposure is considered 0 (card measured by spotmeter and the value considered the right exposure). So if you want to move within the dynamic range of the film the luminance of the set should not exceed 2 1/3 over and 2 2/3 under when measured with your spotmeter. I think that is the simple way to understand the graph. And alsoo, maybe you just misunderstood, 2 1/3 is two and a third T-stop not seven.
Hope I helped you a bit