For best results, I recommend using a fixed schedule for a garden but enable rain skip. The time will depend on the flow rate of your drip hose. I’d run it for 10 minutes and see how many inches deep the water gets into your soil – for garden plants, you’ll want at least 6" watering depth. Then adjust the time. Run in the morning, check soil moisture in the afternoon to see if another watering run is needed to keep the plants from wilting. Vegetable gardens like consistent watering and Fixed schedules does that with none of Flex’s accidental skips because of forecasted rain that may not actually happen.
If you want to use Flex, which can work well with a bit more effort, you’ll need to find out the nozzle in/hr of your drip hose. If there’s some kind of flow rate listed on the hose packaging, that might help to figure it out. E.g., it is possible to convert from GPH or GPM to in/hr by using a representative diameter of a plant’s drip line. However since a drip hose might not have individual point sources of water but waters at every point along the hose… I’m not sure how effective finding in/hr would be. Also its flow rate could be dependent on water pressure so it may be best to measure the flow rate yourself somehow by having it operate over a bucket and finding the volume of water over a 10 minute period and multiplying the volume by 6 to get an estimate of the hourly volume. Ideally, for hypothetical best results, the bucket would be positioned below where the drip hose would normally sit to water plants.
But even with flex you might need an afternoon pick-me-up with a fixed schedule, depending how hot it gets in your area.