Best of luck using the smart watering in the desert. I live in New Mexico and also found that the flex schedule simply did not work for me. The default settings were too far off out of the box and you had to know way too much about its internals to get it to work.
FORTUNATELY, I our local water authority publishes a watering recommendation that really works. They just recommend how many days/week we in Albuquerque should water for different months (see http://www.abcwua.org/Water_by_the_Numbers.aspx).
This translates perfectly to Rachio’s “Fixed Schedule.” I figured out how long the sprinklers needed to run then I simply use the water company’s recommendation to set the interval. I also enable some of the Weather Intelligence settings to adjust for rain and wind. Then I set a reminder on my phone for every few months to tell me to adjust the interval. If I notice my lawn isn’t getting quite enough water, I increase the duration appropriately. This works great, it’s simple, it complies with our local watering recommendations and restrictions, my lawn loves it and it gives me a lot of control.
I see that there’s now a “Seasonal Shift” option which I hoped would make it even simpler. However, looking at the FAQ, I see this shift only adjusts the watering duration (how many minutes it should run) and not the interval (how many days it should wait between watering).