Hi @chengjh1, The Tobii 4C calibration setup is indeed locked into being at the bottom of the screen only as this is the most optimal position to avoid interference for artifacts such as user eye lashes which can seriously impact trackability. Should you need a customised setup however, then it would be necessary to look at the Tobii Pro range of trakers for which some can offer alternative mounting scenarios.
I am afraid that the tracking area is very much dependent on the internal camera of the eye tracker and are designed to operate only within a maximum distance as specified by the screen being used (maximum is nominally 27 inches). Accordingly, there is no methodology for mapping gaze data outside this area scaled back onto the screen although you can of course use any gaze data captured that is outside the normalised screen coordinates and apply your own custom transformation.
Hopefully you can find a workaround for your needs in this regard. Please let us know of any further info we can provide.