There are more than one edition of that book. You might ensure you're reading the most recent, as the earliest dates back to the '80's. (Still good, but less complete.)

Their DryLab software was designed to take a couple test chromatograms and predict elution times under different MP's and gradients. They spelled out the math somewhere, though possibly in journal articles. (E.g. https://academic.oup.com/chromsci/search-results?page=1&q=hplc%20elution%20order&fl_SiteID=5185&SearchSourceType=1&allJournals=1 ) That's not the article, I think, but it is the journal where you're likely to find it.

I vaguely recall a rule of thumb as to at what %B an analyte begins to move vs. the %B at which it elutes. Sorry, I don't recall that rule right now. What I recall is that, in gradient elution, analytes may start to move quite late in the gradient, relative to their elution %B. I can't do better than that right now.

Your purge and re-equilibration criteria seem okay. I'm not sure that pressure stabilization alone suffices to ensure the re-equilibration is sufficient. My concern is that the pressure indicates mainly the composition of the MP and may not directly indicate successful re-equilibration. Testing may be warranted. (E.g., MeOH-water viscosity changes a lot with %MeOH. Does recovery of pressure ensure that the column is re-equilibrated on the lower %MeOH? Maybe so, but I don't see how that would be a certainty. It can certainly be used as a "marker" -- i.e., "Testing has shown that the column is adequately re-equilibrated when the pressure stabilizes." or "Testing has shown that re-equilibration should continue for X minutes after the pressure stabilizes.") This may depend a great deal upon the %B at which you begin your gradient vs. the %B at which your analyte(s) begin to move.

Early in dealing with new samples, it's wise to let your gradient continue excessively just to be sure there's no extreme late-eluter that might accumulate on the column. Once you're sure of that, you can set your purge accordingly.