I know this is an old thread, but I saw this post and wanted to add some comments. The FIRST THING I onted when installing my Rachio 3 weeks ago was that I hated the fact it didn't have an ACCURATE water used calc method. There is an easy way to fix that, and I suggested it and hope they build it in, but I did it the longer way - manually!
I made sure NO WATER was being used in the house for 20 minutes. I then marked my meter reading and ran Zone 1 for 15 minutes, then marked the meter. Then ran Zone 2 for 15 minutes. Etc.
Then, using the EXACT amount of water that left the system for each zone, I created NEW HEADS at the EXACT precipitation rate and basically backed in my numbers. I then ran each zone for 5 minutes again, marking the WATER USAGE and comparing that to what the RACHIO said under GALLONS USED. Ensuring they matched, I knew I was done. Now... forever... unless I change a head, change a nozzle or add a head, I should be pretty darn close.
This could easily be done by Rachio upon install... just ask for the meter reading BEFORE EACH ZONE fires the first time (during setup) - and remove the 90 minute process I had to go through for 6 zones.