Hi Michael, I was composing a reply to your 2nd last post when this one arrived so I thought I'd quickly chime in before a more thorough reply when I have time.
Originally Posted by Michael R 1974
I don't think that I have left the straight line portion of the HP5 curve but I calculate it to be right on the edge and any variations in process variables could push it into the shoulder so my goal in slightly under developing would be to ensure I don't hit the shoulder. I know in the TRI-X case I compared the two extremes of over and under development, but that was to show the effect magnified which could/should still exist (if one has a sufficient amount of overexposure) as one transitions from a lower to a higher curve. Because I am unsure if I have encroached into the shoulder at N dev time, I wanted to reduce this chance of doing so by under developing.
Regarding your comments about HP5's shoulder being different, Ilford have not included any information in their HP5 characteristic curve so we cannot conclude anything but only assume. Even if its shoulder characteristics were different, I would be surprised if the general shape didn't generally follow that shown with TRI-X.
I will reply more about your red lines later tonight or tomorrow morning (OZ time of course !)