Yes I can see that Zimmerman does have an effect, but only on watering duration of a run.
In this paper they stipulate that you shouldn´t increase the Irrigation Duration (as the grass roots can only soak a specific amount of water, more would be overwatering). They claim that the watering frequency should be increased inststead. Well, I am no Irrigation expert so maybe Zimmerman might already ba sufficient, anyway; on the other hand it seems quite convincing what they are saying.
What I would do: I would use the Zimmerman method and the “monthly modification factor” simultaneously (with lower Zimmerman Settings for Temp and Humidity and 100% for rain). So Zimmermann would determine the Irrigation Duration, taking into account the rain and the month correction would determine the frequency of the Irrigation (every n days).
As I said, I am not an expert at all in Irrigation, so this might be plain wrong 🙂