The best way to set up a model with only 1 point of pressure/flow in WaterCAD for Windows would be to define the desired pressure or flow value at that specific point and ensure the rest of the model is properly connected and configured.
A common method of flow measurement is done by using a Differential Pressure Transmitter (also called a DP Transmitter). The Differential Pressure Transmitter often referred to as a Delta P transmitter,
