Hello Dave,
Linear is good, it means it's something in the environment.
Just go to Configure on your sensor and use the Sensor Correction algorithm (see attachment).
The idea is, point to true North, see what the readout on the sensor is, type that number in.
Move to the next direction and repeat.
You have a choice of 4 and 8 points, try with 4 first, it should be sufficient.
Best of luck!
Dragisa 4O9ITT