That is just an approximation. It depends on the enthalpy
of the steam, i.e. pressure of the steam and the degree of
superheat and/or reheat added. A ton/hour of steam at 900
psig and 375C will generate less MW than the same ton/hour
at 2850 psig and 590C. Also depends on the generator
efficiency which is generally assumed to be 80-85%.

no it is theoritical method,
but in real case, we should check the turbine by varying the
speed of the steam and angle of the nozzle,so 1 MW power is
to be generated,at that time we can check the steam velocity
and from that we can calculate the volume of the steam.

1. A single acting stage air compressor with 6% of clearance
volume compresses air from 1.2 bar to 6.5 bar. Find the
change in volumetric efficiency if the exponent of expansion
process changes from 1.55 to 1.72.