If you move your eyes left or right, you lose one or both lines being perfectly "on". In the test shot I posted, that would be CB edge to OB center, and CB center to OB edge. It it is important that left eye is dominant for the left line and right eye dominant for right line. If you angle your face to the cutting edge, that should happen naturally without much thought to it. But it's something you can check during learning process.Describe what "perfect" looks like.