When you are determining the initial watering times for your particular location and vegetation take note of the daily averages for temperature and humidity from a nearby weather underground personal weather station.
For instance you’d like it to water for 10minutes everyday at sunrise, this will be your idealist baseline time. Then the weather for the last week has averaged about 62F and 55% humidity if you exclude “outlier” days. Please note it is important to not include weather that should have been adjusted for. Now use the temp and humidity you found for your location in the adjustment settings with the program set for ideal times. As the weather drifts from your measured data it will adjust the time more appropriately.
The important thing to remember is the Zimmerman method needs to be set for ideal conditions initially and may take a couple of tries to nail down. Once you’ve tweaked the settings it will down a pretty good job of making adjustments for weather that would be considered abnormal, like high temp low humidity, or random afternoon thunderstorms. It isn’t perfect, but, it’s trade-off is in simplicity of setup. Just check on things once in a while and you’ll find you won’t have any problems after those first few tweaks.
I spent a while setting up an ET method that is a lot more complicated and it probably just barely outperforms the Zimmerman method in most locations. Where I live in Colorado I did see some nice water savings with ET, however, virtually no where else do they have the incredible unpredictability and crazy swings in weather patterns that we get here. End of the day, I’d recommend the Zimmerman method to anyone, and I’d encourage them to do the small amount of homework needed to see the gains they expect.