Hi @treep, the centre of the tracked head is in fact defined at the centre of the ‘track box’ that contains the user head and so will vary from person to person (and across environment) and so therefore does not have any particular biological correspondence. In any event, it may make a realistic rendering tricky if your pivot point is at the centre. for example, a nodding action would look unnatural if the axis is centre based. You would need to make some estimations to make the head rotations (x,y,z) around the centre translate into a neck-based pivot point for a more realistic representation.
@refuzee, yes you are right in that the mounting brackets for the 4C (And other Tech Trackers) are designed in such a way to ensure the angle between the tracker and the screen is constant. Therefore any intentional changing of the angle will results in a reduction in eye trackability.
Please let us know if we can be of any further assistance.