on steam seal calculation (steam turbine)

When I calculate self sealing load point on 500MW steam turbine, I couldn't assume inlet of N2 packing pressure & enthalpy.
At this time, I assumed the pressure as HP bowl pressure and the enthalpy as average value between HP bowl enthalpy and HP 1st stage shell one.
but I couldn't believe this assumption because there is no steam balance hole on HP 1st Nozzle.
If there is steam balance hole on HP 1st Nozzle, I think my assumption is reasonable.
I couldn't derived a reasonable value for inlet of N2 packing without balance hole. How can I get the reasonable value for self sealing load point calculation?
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