There are 2 basic approaches to measure or calculate water usage - using precipitation rate x area, or using gallons per minute. Both approaches can be either measured or estimated using manufacturer's nozzle specs (perhaps adjusted if you know how much each nozzle was adjusted).
Of course a measurement is better than an estimate. I have a water meter, but the city reads it electronically every 4 months, so I can't get a real GPM measurement from my meter.
I would only go to the trouble of setting up catch cups to measure precipitation rates if I wanted to adjust my nozzles, not just to measure flow for rachio stats. However, it might help me tune my flex schedules.
So, looks like an estimate for me. None of my zones are perfectly rectangular, and none have all heads with the same flow rate. Many of the nozzles are variable arc, so the flow depends on the arc. Some heads may have had radius reduced, so flow would be less. I can estimate the GPM flow from each head and add up the total flow for the zone.
I am not a fan of having the precipitation rate defaulted by nozzle type, instead I would enter it zone by zone. I bet that most zones have odd shapes, irregular head placement or imperfect head coverage, so the manufacturer specified PR is not achieved. However, the efficiency for the zone can be reduced for irregular zones.
I think the product suggestion is to have total zone GPM, Precipitation rate and zone area as 3 zone parameters. 2 must be entered and the third is derived by Rachio, using the other 2. The user can choose which 2 will be entered, and which will be derived.