The usual way of representing the steam consumption of a turbine is the willans line, for example...
Often, a willans line for a specific turbine is not available, and one only has has incomplete information about an installed steam turbine; namely the rated power and (perhaps a guesstimate) the isentropic efficency or specific steam consumption at full load. But what you need and don't have is the steam consumption at the turbine operating load.
Isentropic efficiency η by definition is given by
η = (hHP - hLP) / (hHP - hLPisen)
We define the ratio
m = ηFL/(1 - ηFL)
where ηFL is the turbine isentropic efficiency at full load
The isentropic efficiency η at load P can be estimated by
η = m·P / (PR + m·P)
where PR is the rated turbine power.
The specific enthalpy at the turbine exhaust conditions is then
hLP = hHP - η·(hHP - hLPisen)
The steam consumption at operating load P is given by
ms = P / (hHP - hLP)