Back in 2016, I used a combination of reading the meter before/after running a zone and Lawn Watering Guide - Interactive Watering Guides - Water Use It Wisely to figure out how much water I should be putting down. We had a long thread on this, and there’s another recommendation a couple of replies down from my original post on this thread.
Here’s another method: GPH nozzle to in/hr conversion for single point drip emitters
And this might be the best resource for you:Drip Emitter Calculator for Precipitation Rate & Area