Ummm, not quite what I would come up with, but I find this guide VERY effective for calculating water needs. If you would like to walk thru how to use this calculator, I can help, but @azdavidr did a FANTASTIC job explaining the procedure.
But, I need to reiterate…if prior to Rachio you watered multiple times a week for shorter durations, you might find your plants struggling, so might need to tweak some things in order to “re-train” your plants.
