This is very hard to do. Every time you get a pulse, the amount of water used changes. But you’ll never know if you’ll get another pulse quickly or not for hours.
There’s an app [APP] Insight Trends that might help you to do what you need. It uses the insights to detect for instance rapid changes. I think it will allow you to trigger on the things you wish, even though it may not give you the actual flow rate per se.
