Bill, perhaps Clerc is referring to the equivalent print tonality that appears the most grainy. Studies have been done on this. I don't have the data in front of me but I recall discussion in Richard Henry's book. For example, assume you have a negative of uniform density (any density), and you print a test strip of it on paper. I will check the book for the reflection densities, but I recall it was around what we would normally think of as somewhere between zone III and IV that the tone appears the most grainy to an observer. Perhaps Clerc is referring to a negative density that would normally print to the reflection density at which graininess is most pronounced to the viewer. Just throwing out some ideas.