We are both attacking the problem from different angles and arriving at the same result.
Your solution is quite accurate, works with all setups(overall better, I agree), but requires a flow meter…something that is impractical in my setup (I personally would have to place a meter on every secondary line as there is no accessible space on the primary line feeding them).
My method involves calculating volume or really precipitation rate(volume over time), this is quite the pain for subsurface if not impossible, and less accurate for drip systems, but works well for my setup, sprayers and surface drip. I suppose one could cover the sprayer and funnel the water into a container and measure direct volume of water output, versus the tuna can in the middle, I just used the manufacturers stated precipitation rate and made sure I had enough water pressure to keep it spraying at full capacity.
End of the day it sounds like we have 2 simple solutions with their own pros and cons, that provide the information needed to adjust the watering time.
I use an ET calculation to determine how much water is needed and multiply that by my calculated time to get 1mm in the total zone area, you would be doing the same with a measured time to get 1mm in the total zone area. Any rain can be subtracted from the ET calculation, so the weight of the rainfall is figured out already. If we want to water more or less we can just lie about the time it takes to get 1mm and it applies the adjustment percentage more or less.
Anyway we’ve spent plenty of time saying the same thing different ways, your example above math wise is exactly the same as mine (Volume per Time) / (Coverage Area) = Depth per Time. Which gives us a way to calculate either, depth or time needed, quickly using a computer. So my friendly (I must admit quite patient), neighbor to the North thanks for the discussion, it’s interesting to see how different people work out problems. BTW no bother on the Imperial and Metric mixing, I’m German and live in the US so either system is fine.