Abstract: A built-in calibration sub-system of, or an auxiliary device to be used with, a normally closed water supply control system of the type that uses a single pressure sensor and a pressure decay versus time measurement to make volumetric flow determinations for water supply control purposes, including leak detection and flood risk mitigation is provided. The present disclosure provides a system that uses an auxiliary electrically actuated valve and an orifice to cause a controlled flow of water to discharge from a plumbing network over a pre-determined pressure range such that the normally closed water supply control system's microprocessor can calculate the calibration factor it needs that relates change in pressure over time to volumetric flow rate. Methods of calibration and testing of the leak detection feature of a normally closed water supply control system are also provided.