So, is this 0.35 variation a linear/across the board difference?
It's roughly linear, but it isn't a constant. This difference is for mash temperatures. It's larger at sparge temperatures (maybe 0.1 more). The difference is 0 at room temp (68F, or whatever they were calibrated at). And this is using water. I've seen some (credible) sources say the variation is lower when using a mash rather than water (maybe 0.2).
I’m very interested in the original source of this number. If it’s based on the pH change in plain water, which is caused by the change in the water’s dissociation constant, then the 0.35 number is not going to be correct in wort. This is because the pH of wort is determines by the dissociation constants of all the various acids that are in there.
Briggs, et. al. mentions a 0.35 shift at 65C and a 0.45 shift at 75 C.
Note that the 0.35 number may reflect both a change in mash pH as well as a shift in the enzyme’s pH optimum. Testing cold and hot wort with a temperature correcting pH meter only shows you the actual mash pH change.
BTW, congrats on your book that’s coming out soon.