the X2 eye trackers cannot be used upside down. And they are designed to track eyes from below, looking up on the face. So that’s why the eye tracker needs to be mounted below the screen.
If it works better in your scenario there is a Tobii X2 Desk stand that you can use to mount the eye tracker.
For more information about the track box of the X2 eye trackers we recommend to read the “Tobii Analytics SDK Developers Guide.pdf” included in the Tobii Analytics SDK package.