There is a great article here detailing how we estimate usage, and how to use that GPM you calculated to work back into your zone settings and tune in your usage values. If you have any questions on that, please let me know. I agree that adding the GPM per zone would be a useful feature, however it is not a full fix to our problem, because ultimately, we need to know how that amount of water is being distributed over what area. There is a good thread here detailing the pros and cons of different ways of approaching the zone inputs.
If you would like, I can clear out the estimated gallons used for you whenever. Just give me the green light and I will reset that for you!