This post gives some guidelines on how the Tobii eye trackers should be mounted for good tracking performance.
The user’s eyes must be within the track box.
The track box is the volume in which the eye tracker is theoretically able to track the user’s eyes. The user may move her head freely and still remain trackable as long as the eyes remain within the track box.
The size of the track box depends on the eye tracker model, so please check the technical specifications for your eye tracker.
The tracker should point towards the user’s eyes from below.
If you mount it from above eyelashes and glasses might get in the way. And mounting it upside-down will confuse the eye tracking algorithms.
The tracker must be in the same orientation as the user’s eyes.
It doesn’t have to be perfectly aligned with the user’s eyes, but it shouldn’t be perpendicular. For example, mounting the tracker on the lower edge of a tablet in portrait mode and then tilting it to landscape mode won’t work.
Avoid large gaze angles.
The gaze angle is the angle between the eye tracker and the user’s gaze point. The Tobii eye trackers work best when this angle is below 35°. (The exact figure might differ between eye tracker models.)
This is one of the reasons why eye tracking is difficult with very large screens. The other reason is that people tend to stand back or lean back when the screen is very large, and so they might drop out of the track box.