I've used several different anchors over the years and the most consistent, for me, is with the middle finger on an upper tooth that allows my thumb knuckle to just graze the back of my jaw bone. These two positions don"t move, in relation to the eyes, if you move your lower jaw. Works split finger or three under.

It really doesn't matter where you anchor, as long as it is comfortable for you and consistent. If your anchor and your grip aren't consistent, your arrow placement won't be.