I was playing around with getting deep desert watering using low PR values, and noticed an apparent discontinuity. I decided to look into it further and I now see the issue. I use 0.4 in/hr for three 2 GPH emitters on each of my trees. Since I also have single emitter shrubs with 1 GPH heads I figured I could just use 0.07 in/hr for a custom nozzle for those shrubs. The results weren't as I expected. Tonight I went in and charted duration vs. PR and see that the total duration is capped at 8 hrs, and I see now that at about 0.12 in/hr the 8hr cap is reached. After that the intended target of 0.96 inches is not met, and the watering interval starts to change. It seems that at minimum some sort of user warning should be given, and perhaps the PR should be forced to 0.12 in/hr so that the interval stays fixed as well.
Here's the data I took.