I’ve tried to do the same thing (but a few versions of the Unity EyeX ago).
Anyways, what I did was using the eye positions (you get this is mm from Eye Tracker, I think) together with screen size (mm) and screen gaze position (normalized coordinates) to calculate the direction of the eyes. Thereby I could do manipulations on the camera.
Maybe it is possible to get the ET -> Screen offset from the calibration to improve the result.
However, my final implementation did not get the result I wanted due to lack of time (and I knew that a new version of Unity would come that would make a lot of my code much easier and prettier with in a week or two of when I worked on it).