Take some measurements using : Water tank, bypass valve, known volume container and stopwatch.
Adjust the bypass valve to an almost closed position that allows slow flow.
Measure the time it takes to fill the known volume container.
Calculate the flow rate and take note what the sensor recorded.
Repeat with different valve positions. You will have different flows.
At all times record the flow and value indicated by the sensor.
At last, you will have a straight line that will allow you to establish what your factor.
Sorry: don't use the 4.5 factor with this method, use 1.