@Peter I’m still using historical weather data from WU as a weather source forwarded by my Netatmo PWS to WU. My OpenSprinkler runs in Manual Mode and will be updated via the OS API interface. My ESP32 NodeMCU program does the Zimmerman calculation based on empirical weather data retrieved from WU. My base parameters are 70°F, 65% humidity and 0.0 inch precipitation. My geo coordinates are 49.47369, 10.94380. My WU station name is IFRTH65. The location name shown in OpenSprinkler is Burgfarnbach a suburb of Fuerth, Germany. This seems to be the location of the nearby OWM weather station.
I don’t know how the actual OWM based OS weather adjustments are working in detail. Reading forum comments I thought that there are still variations during night and day not caused by any rainfall.
It would be nice if you can provide more details or run a comparison.