Have you called the manufacturer?
Have you read their (original) tech manuals for the steam conditions at each stage extraction and condenser temperature at 100% power?

With that information at each stage at 100% power (or from the plant's MEASUREMENTS at 100%, 90%, 80% power, run it for an hour just before shutdown at 50% and 60%.

Heat and Mass Balances at part load conditions are complex. These calculations are usually done with specialized software like "PEPSE" or "GateCycle". Due to their specialized nature, licenses for these programs are very expensive. In addition, considerable details of the components in the cycle are required as input to these programs.

However, if you have the manufacturer's Heat and Mass Balances at part loads you may be able to plot various parameters like isentropic efficiency vs throttle flow so you can calculate performance at part loads. Once you have these plots, you may be able to develop an Excel spreadsheet to assist with calculations. Even then, you will have to iterate calculations for various components to get agreement between components.

Best of luck!

stgrme is correct- use a canned computer program, provided you correctly specify the condenser size, circulating water temperature, turbine inlet valve position, in additon to the full load turbine performance parameters.

The exhaust pressure is not predictable unless you know the condenser sizing and circulating water temperature, whihc is a function of the ambient air temperature and the wet cooling tower size or ACC performance parameters.

The part load performance is also dependent on the turbine inlet valve position ( constant pressure or variable pressure) and the iinlet steam temperatures ( which vary as load drops , depending on boiler characteristics).

At very low load ( < 15% MCR hot reheat steam flow) there can be extra losses at the L-1 exhaust blades due to "recirculation" or aerodynnamic instability.

All of the above considerations are usually modeled in the canned programs.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick