Q: Frank writes: "I would like to know how many eyes should I be able to see in the Edge mirror? If I can see 2 eyes, then my head is behind the ball. Is that correct?"
A: Great to hear from you - terrific question, too.
There are a couple of things working into your eye position:
1. Ideally, you would like your dominate eye over the top of the ball. If you are right eye dominate, then your left eye would be slightly in front of the ball and out of the mirror. Conversely, if you are left eye dominate you would see both eyes. (I am assuming you are right handed :)
2. You are correct about seeing 2 eyes and your head being behind the ball. The main thing is to make sure both eyes are on the line of the putt. You don't want your eye "twisted" so that one eye is inside the line and the other eye outside the line. Your perception would be adversely affected.
The next time you are practicing with the EDGE Mirror, experiment with the ball position. Move it forward and backward and watch how your eyes see the line. You will find your best position.
Let me know if I can help further.
All the best,