There are a few paramters required to caclulate the turbine efficiency.

Actual plant power generation rate
Steam Flow rate from boiler
Specific Steam Consumption
Steam Pressure at Turbine inlet
Steam Temperature at Turbine inlet
BFW Temperature
Enthalpy of Steam at Turbine inlet
Enthalpy of BFW
Steam extracted from the plant
Steam Pressure at Turbine extraction
Steam Temperature at Turbine extraction
Enthalpy of Steam at Turbine extraction
Steam to condenser
Steam Pressure at Turbine exhaust

I have a spreadsheet to allow you to plug and play your data to calculate the efficiency. Check the spreadsheet formulas to determine how to minipulate the data. send me a Private message for the spreadsheet with your email information. for some reason, i cant upload the .xls type file as a message attachment.