A turbine valve control system is provided. The turbine valve control system includes a variable flow metering device, a first sensor, a second sensor, a third sensor, a fourth sensor, memory, and processing circuitry. The variable flow metering device is capable of meeting a plurality of predetermined mass flow rates by varying positioning of the flow metering device. The processing circuitry is configured to receive the signals, determine whether the flow is subsonic or sonic, and implement a corresponding one of the first and second computer program codes to calculate a combined coefficient of discharge times area that will generate a desired mass flow rate for the flow metering device. A method is also provided.