Well i was always told that 1250 is max after the turbo.
The egt is hotter...way hotter before the turbo.
My truck always was ran on the ragged edge when my father drove it.He kept it at 1225.Our thermocouple is after the turbo. J&S diesel(who did all the hop-ups)and Cummins informed him to keep it under 1250 post turbo.