When this discussion came up previously, I think we concluded that different manufacturers measured differently. There were one or two that seemed to use the spine-to-hone-wear measurement, but most just seemed to measure edge to spine.
I always measure edge to spine, as it's the most straightforward. I've seen 6/8 wedges that would have to be called 2/8s if you used the hone wear approach.
To me the question is, what are you trying to measure? To me, the measurement should convey the heft and width of the razor. Since a significant portion of the weight resides in the spine, why not include that in the measurement? It's not like that extra 1/16" or more of width would go away when you went to shave just because the hollows don't extend that far...
Josh