When first learning to cue, many people get in the habit of cueing the side vowel positions and the throat vowel positions too far away from the face. Cue positions far away from the lips are not as visually sharp and clear to the viewer as those closer to the lower face area.
Human eyesight is subject to foveal vision.
This means that we actually see adequately lighted objects much clearer at the center point of our vision than we do above, below, or on either side of the foveal ring. To demonstrate, concentrate your vision on one word in the center of this screen. Your lips become the center of their foveal vision. Cues at the side of the mouth or on the chin are not going to pose a problem because they are well within the primary range. Cues made far to the side or far down the chest cannot be seen as clearly because they fall outside the primary range of foveal vision.