Would another way to do this be to work towards the ratio of what Rachio says versus what you observe? Here’s what I mean . . . Rachio said I used 149 gallons in 12 minutes, roughly 12 gallons a minute (I rounded). But I measured 5.25 gallons a minute. Rachio also set my nozzle inches per hour at 1.5. So, 1.5/12 = X/5.25, right? Where X = the more accurate nozzle inches per hour, reflecting what I measured by gallons per minute in the field.
Assuming the rations are algebraic (?) and not some other dimension, that means about .65 nozzle inches per hour.
I’ll have to check after the zone gets watered to see if the new water usage reflects what I predict with my measurements.