There is a lot more to it than the original post describes. First, you cannot admit steam to a steam turbine unless you are certain the steam temperature is within allowable limts, so opening the turbine valves prior to starting the boiler cannot be permitted.
Second, there needs to be speed control of the turbine prior to synchronization, so opening the turbine valves without speed control override can only be done once, and then you would need a rbine, and a new pair of underwear as well.
I think the topic is better described as "variable pressure operation" or sliding pressure operation of the turbine ( and boiler). The optimum method is a function of whether the tubine uses arc admission ( siemens and westinghouse) or partial arc admission ( GE and Hitachi) of the inlet throttle valves. Also, the type of turbine bypass avaialable affects the startup mode ,and also there is distinction between admitting steam to the IP turibne follwoed by transfer of laod control, or simply admitting steam to the HP stage first. And finally, the type of boiler you have also affects the startup procedure, some differences being a constant pressure style drum boiler, a variable pressure style drum boiler, a constant furnace waterwall pressure once thru steam generator, a varible furnace water pressure steam generator, and there are other differences.
In any case, once you have synchronized the steam turbine, the pressure at the superheater outlet will vary linearly with steam flow if the steam turibne valves are left in a fixed position.
