Nice! I have been getting unreliable results from the default service since it moved to OpenWeatherMap, with watering levels close to zero when my stations run around sunrise, like this thread.
Just a little gotcha, which I only worked out from reading the source, that the files have to be served from http://web-server/, no HTTPS, no subdirectories. I had to put some symlinks into my web server at the top level to point to my weewx installation.
Zimmermann adjustment works well enough for me, but weewx knows about evapotranspiration. I might have a play with that.