That logic is already included in the firmware. Here is a line of code that implements that:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/main.cpp#L443
it’s not user configurable yet, but it basically says if the watering percentage is less than 20% and if the calculated watering time is below 10 seconds, set the water time of that zone to 0.