Hi Shane,
there are two steps to get this to work. The first one is to set up the “active display area”, essentially telling the eye tracker where the display is relative to itself.
The second step is the user calibration where you show visual stimuli on the screen while recording the user’s gaze, so that the tracker can compute an accurate gaze point. You can use a standard calibration procedure for this, you don’t need anything special.
For the REX there is no nice GUI tool for setting the display area like there is for the TX300, but it’s still possible to achieve the same result yourself with a tape measure, some trigonometry (maybe?), and the GUI-less setdisplayarea tool which can be found in the tools directory in the Gaze SDK packages.