If you are experiencing an offset or if some parts of the screen (especially the four corners) are not intractable, the emitter needs to be adjusted so that the IR curtain is parallel to the surface. In most cases, the emitter might be placed at angle such that the IR curtain is pointing away from the bottom of the screen causing interaction "dead zones" at the bottom of the screen.
One way to make sure that the IR curtain drops parallel to your surface is to
1. Tilt the emitter until you see a white band across the middle of your screen as shown in the picture below. This means that the emitter is placed at an angle such that the IR curtain is hitting the center of the screen.