extraction steam turbine

Calculate the theoretical maximum by flowrate and enthalpy data at inlet and outlet.

Divide the measured horsepower of unit by this number.

You could play around with this free software - you will have to guess at an efficiency and narrow it down to get your know proper flow conditions.

http://www.katmarsoftware.com/turbine.htm

If you have a one stage turbine it is more likely a back pressure machine. Extraction flow is arrived by throttling steam downstream of the entrance with a second set of valves.

Keep in mind that any calculations you make are only as valid as the instrumentation. The installed instrumentation on older equipment is usually inaccurate, new installations are suspect.