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.
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%.