Exhaust steam desuperheating

Hi, everybody!
There is a rule of thumb or best practice for steam desuperheating: from exhaust steam of 150°C to 128°C, what is the maximum practical temperature of the injection water?
Dave, I understand that it depends on the specific application, but as rule of thumb, how many degrees less? closest is better for flashing-mixing, but demands more water to lower the steam temperature.
the heat required is so much less that that needed to vaporize the water, it is only a minor correction in you heat balance calculation. The challenges are in you piping arrangement.