In some aspects, a steam turbine system includes a high-pressure turbine section; a low-pressure turbine section; a high-pressure control valve operable to provide an adjustable flow of steam into the high-pressure turbine section; a low-pressure control valve operable to provide an adjustable flow of steam into the low-pressure turbine section; a controller associated with the high-pressure control valve and the low-pressure control valve. The controller is operable to: receive measurements of three or more different operating points of the steam turbine system, the measurements of each of the three or more different operating points including a position of the high-pressure control valve, a position of the low-pressure control valve, and two of process variables of the steam turbine system; calculate coefficients of a steam performance map of the steam turbine system based on the measurements; and generate the steam performance map based on the coefficients.