Bruce: You are clearly more experimental than I am! I've been using PMK for something like 20 years, always at 1:2:100, and with relatively minor changes to the recommended times, based on my own testing. PMK does follow an expected time/temperature curve for contrast. My "normal" time is about 1 to 1.5 minutes shorter than the times in the book, because I was consistently getting negatives that were more contrasty than I liked. Hutchings mentions in his book that the curve flattens at the 20 minute mark, developing beyond that adds no more contrast. I've never come close to using a time that long. As for proportions, the only one mentioned in the book is 1:2:100, so that I all I've ever used. (As an aside, since you don't mention the format you use, I've been very happy with PMK for 4x5 and 120 films; for 35mm I prefer D-76 1:1.)