If you want to add a cutoff for the watering percentage, this is probably the best place:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/weather.cpp#L84
this is where the watering percentage received from the weather server is parsed and saved.
If you want to add a cutoff for the actual water time, here is the best place:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/main.cpp#L401
this is where the station’s water time is multiplied by the watering percentage, and scheduled into the work queue.