The device accuracy is probably 50%
Get a 5 gallon bucket
Take a hose and let water trickle through. The sensor
Record. The volume
Then repeat at maximum wTer pressure
Compare readings.
The best you can hope for is to have a calibration point or curve at on flow rate and another for each flow rate.
Go look at pump data sheets they have multiple curves based on rate of flow
In addition the geometry of the pipe entering and exiting will affect the readings you would want to have a straight pipe in and a straight pipe out for at least 20 diameters look at gun caliber to get the idea whatever the pipe size 20 * that so if its 20 mm diameter you'd want it 400 mm long on the exit it should be half that distance or longer you can prove that by using a hose and bending the hose to a curve or holding the host straight and see how the readings change
You can develop curves for the sensor but there is no single value