I ended up doing the math to calculate how much water gets laid down. I knew how many degrees of arc each rotor was covering, I knew my gallons per minute, and I knew the range of the rotors. I got about 0.25″ per hour.
I didn’t totally trust this, so I bought a pack of catch cups from Home Depot ($16.99 for 12), set them up, and got almost exactly the same result. 🙂
My coverage uniformity was anywhere between 50 and 75%, though, so I’m considering switching out the rotors for a different brand or nozzle configuration.