With drip systems, you kind of have to throw the chicken bones. Those systems are design to be tweaked at each irrigation site as shrub areas have heterogeneous root systems.
I set my custom head to .5 inch/hour (I use only 2gph emitters) and my root depth to 10 inches. This gave me the runtime I wanted at 45 minutes (bigger plants get additional emitters).
My allowed deplition is set to 50% and I adjusted the watering frequency by increasing the transevap co as I’m on a flex daily schedule.
Again, it’s all chicken bones, as I have a few plants that can go 1-2 weeks without water and I have a few plants that require water every 2-3 days at peak heat (hydrangeas in full sun), not to mention, my soil sucks out loud.
So I know how much water each plants need and I have based my zones on a 45 minute run time and backed my controller into it.
I wish I could give a mathematically sound answer but I have not figured that out yet, way too many variables in play.