I would also like to ask a follow-up question:
how do you know the center of the screen considering the screen can be angled with regard to the tracker. Is this something that should be totally fixed according to the way of fixing the tracker to the screen (i.e. predefined angle) or is the screen when beeing angled affecting the center of the system.
I guess what I am asking in clearer words is: Is the coordinate system just calculated on the basis of the tracker in world space or on the basis of screen/tracker relation in world space.