It’s not difficult to add min/max for watering percentage — these will just add two option values and will definitely fit in the current EEPROM.
If you have some programming background, it’s actually very easy to modify the firmware code to add the limit. Basically this is where the firmware gets the watering level returned from the weather call:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/weather.cpp#L75
By adding one extra line of code you can clamp the value to any fixed min/max value of your choice. Of course the downside is that the min/max values this way cannot be modified on the fly. But if you need it in a hurry this will be the fastest solution.