Hi @cockcomputer, unfortunately there is not a great deal one can do to improve accuracy of reported eye position beyond the following environmental considerations:
Ensure eye tracker is not inline with direct sunlight or other strong sources of Infrared radiation (such as open windows, spotlights, etc), Reduce ambien lighting in the computer environment if possible. The presence of thick glasses, heavy make-up or facial pearcings can also interfere.
After running the user calibration, how does the accuracy test look for you (ie the 9 circle test). Also the gaze overlay feature included in the Tobii Core software might be useful to see if there are any inherent issues in parts of the screen.
You might also try on a another user to see if the deviations are persistent.