I recently did a GPM test (bucket test) and only got 9.7 GPM seem low? This is from a well and a new pump, I filled the bucket with a water hose coming from the pump (if it matters). PSI was 45-50.
I redid the bucket test after finding a fitting I did not snug up very well and instead of using a waterhose I went straight from the spigot. I got better numbers 13.6 GPM. .
When I did my GPM bucket test at the spigot I got 8 GPM. Rainbird told me it was to low. So I re-tested it in the basment right at the poly pipe off the meter, and I got 30 gpm. The spigot was reduced down to 1/2" where the main line was 1" and there were no elbows or valves to slow it down.
I think it would be important to test it right where you would connect the sprinkler main. I would test it closer to the pump and use the biggest line you have (no reducers)