Shawn, thanks for sharing your knowledge. You are right and I see my first adjustment (humidity baseline 65%) as a starting point.
I have started last year with an OSBee controlled by IFTTT and apilio.io applets: weather-controlled-sprinkler-system-with-osbee/. I learned from this expierience that the weather forecast is not the best way to control the water times. For example, we have had a lot of afternoon thunderstorms in the last two weeks, resulting in heavy rainfall in nearby places, but no rainfall at all at my home. This means Zimmerman or the ET method are a much more better solution to do sprinkler control than using weather forecasts.
For the time being I suffer a little bit from the changed Weather Underground policy of not supporting my Netatmo station anymore. I am still investigating nearby weather stations that are still reporting to Weather Underground, but it takes some time to find out if I can rely on their data. That was also the reason for me to make use of my own rain gauge to pause any watering in case of more than 5 mm rainfall measured at my home location.
It is good to hear that the Zimmerman method needs to be setup for ideal conditions. I will follow your advice and try to find out what are this ideal conditions for my place and what parameters need to be tweaked to make the best out of the Zimmerman method.