Flex schedules have a constant duration for each zone (which you can adjust slightly in the schedule itself) and just the watering frequency changes. They do take into account daily temperatures (including humidity) which subtracts moisture from the zone. When the zone is depleted, we water. The basic theory is that in hotter periods (more evapotranspiration) the frequency increases, and with cooler periods frequency decreases.
Here is some more literature to help out:
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