@Tom & David
I agree with you both.
The difference is the same between a measure and an estimation.
Both of them can be useful depending on the aim or the available equipment or money.
I also would like to have a flow meter since I had the very same problem which Tom had, but in the meanwhile I’m using the estimation method. But I did the estimation in a different way: I let each channel run for 5 minutes and record the amount of water on the analogical main flow meter of my house.
No need to say that all the other tap valves in the house were closed.
From time to time I repeat the procedure to check for flow consistency