Stephen - I've gone through the Nelson/Simonds and Delta-X papers and I have some concerns and questions which I will try to post later tonight when I have some time. In particular the print judgement speeds in the figure with the six hypothetical curves surprised me at first - which leads me to more questions concerning 0.3G. And the papers really raised some fundamental questions for me.
As you know based on my many previous posts on the subject, I think Jones had it right when it comes to the underlying concept. Local contrast is what matters, not a density. Which is what led me to start this thread. How that principle evolved into practice though, I'm still not sure I "agree", which may ultimately help to bridge the gap between ISO and ZS-style personal EIs.
For now I'll leave you with one quick question. How did an extension of the straight line down to fog density end up being called Inertia Speed? What is the basis for that construction? Is it essentially a speed point such that there is zero toe/compression relative to the maximum gradient in any development scenario?