0-2500mmWC is the pressure drop across a primary flow element, it's not a flow rate. You need a basic flow rate to convert to another flow rate.
0-2500mmWC represents some calculated flow rate, depending on the design inlet pressure, design temperature and characteristics of the flow element. Without knowing what flow rate at 0-2500mmWC DP was calculated to be for that particular installation, there's no way to convert to any other flowrate.