Maybe you should try some books dealing with straight sensitometry. Photographic Materials and Processes, Sensitometry for Photographers, Photographic Sensitometry, or The Manual of Photography are all good serious books on the subject. IMO, Davis likes to do little short cuts to make things easier. Short cuts can also leave out important information for a better understanding of the subject. I feel it's better to understand why you do something than just know how to do it. The books listed above will help with the why.