I normally don't recommend flex for watering drip unless you truly know the precipitation rate coming out of the emitters or drip system. Also, depending on the crop type chosen, flex will want to run considerably longer on trees or shrubs since they normally have a longer root zone depth. The combination of much lower precipitation rates (drip) and long root zone depth will have it watering for quite some time (correctly, for a deep watering). If you know the root zone depth of what you are watering and the exact precipitation rate it will work better. Flex works well for turf type crops with normal spray, rotor nozzles but can falter when dealing with drip and trees/shrubs, etc.
Hope this helps.