It is nice to know my wild assed guess is not so far off the mark. Diffusion analysis is a way to measure the exposure of a scene as well. I usually reserve diffusion for use with my Colorstar when there is an outdoor shot with one no big colour bias, and no obvious spot for metering as neutral grey. It is usual that a general outside scene intergates (ie diffuses) to neutral grey. The remarks for calibrating the paper speed still appear to be correct.