The uncalibrated temperature on the Duet 2 (Duet WiFi and Duet Ethernet) should be within 13C of the true temperature. For the Duet 06 and 085, it could be up to 45C off. To calibrate it:

1. The whole of your printer must be at room temperature. So do this after leaving the printer turned off for a long time, e.g. overnight.
2. Turn your printer on
3. As quickly as possible (before the Duet has time to heat up), connect to it using Duet Web Control and read the temperature from the CPU Temp. box on the Machine Control page. Call this Tc.
4. To determine the current room temperature, read off the extruder heater and bed temperatures and take an average. If you have a PT100 sensor on the extruder, use just the extruder temperature because it is likely to be more accurate. Call this Tr.
5. Send command M912 P0 S# where # is (Tr - Tc). It may be negative. Assuming you did not already have a M912 command in config.g, after a few seconds the CPU temperature should then read about the same as room temperature.
6. Add that M912 command to your config.g file. It doesn't matter where, but I put it near the end, before the M501 command and the T0 command.