The Flume device is very accurate. That is, the water usage that Flume reports will agree very closely with the water usage reported by your water meter.
We had an independent lab test the Flume device on a variety of different water meters (Neptune T10, Sensus SR2, Hersey 420 and Badger 25).
In most conditions, the Flume device agreed with the meter within +/-1%.
Below are the results of a test recently performed by Utah State University: