Thank you for your answer. As I was on the phone, I was unable to verify the number when I wrote my posting, so I was off way off (oops), missing decimal.
So, the high alerts I am getting for 2.4, 3.5 and 3.9 and 6.0 gpm (different zones).
The sprinkler guy who was with me when we calibrated, was astonished that 4 zones showed such low numbers (3.5-4.5 gpm). One zone even only showed 2.1. One zone with only 3 heads seems to fluctuate between 3.5 and 6 (taken from emails I got from the last watering).
I’ll post pictures and more data to answer more specifically to your points.