Hot end temperature calibration (no replies)

Hi all,
i am trying to dial in a new hot end, the host tells 40 deg my thermocouple measures 20 deg
i have the correct thermistor selected in marlin and its fitted well with heat sink paste
i have run autotune for the pid
i am using a mendel variant with melzi board
all works very well, just does not read the right temp. grrr
does anybody let me know if i can adjust the room temp manually in marlin
i dont realy understand how to use the "createTemperatureLookupMarlin"
any help on this subject would be fantastic