Hi Philippe,
The EyeX Engine speaks in pixels on the Windows Virtual Screen (the one that is illustrated when you open the Display Settings in Control Panel/Settings Window). The Virtual Screen correspond to physical monitors attached to the PC.
If you want to use Tobii eye tracking without a monitor, you need to use the low-level Tobii Gaze SDK instead, and manually set up a virtual screen plane and implement some kind of custom manual calibration procedure for it.
See this reply also: http://developer.tobii.com/community/forums/topic/tobii-eyex-without-pc-monitor/#post-3208