I'm still not sure what you're getting at - ie what sort of use to put it to. Re the equation, I learnt it from introductory Kodak sensitometry publications in which there is some brief discussion of the principles, units, that sort of thing. But I never really dug into the details. Perhaps that's why I still can't figure out how to do a proper film speed test with a contacted step tablet (in the camera or on the enlarger baseboard) without a calibrated sensitomter.

Then of course there are the usual issues related to the potential differences between the metered exposure (or the exposure set on the camera) and the actual exposure at the film plane.