Refer to this topic: Need some help with Nozzle PR settings
I have the Hunter PGP and Pro-Spray nozzles. Those GPM’s are probably by spec but how they were installed may change that.
You can do one of two things: catch cup test or water meter. Luckily my sprinkler system is on a separate meter and I was able to figure it out via the water meter.
Also, take a look at this topic: Catch cups vs Water meter
Those should help you out!