If the putt is "a little left to right" then, presumably the mirror is not laying completely level as the ground is sloped. Checking with an adjustable shaving mirror, just a tiny angle change will move the eye reflection a couple of inches over 5 feet. I think these mirrors can actually be quite misleading unless placed on a completely level surface. Good for home use, but be careful with them on a contoured putting green.

Malaska has a drill where you place a tee all the way in the ground about 3 inches in front of of the ball on a straight putt. In that drill you should put your eyes where the tee and the hole are in a straight line. Similar to this. Perfect is wherever your eyes start to see “reality”. There not much debate. Everyone’s eyes perceive differently and there are different shaft angles on putters. Finding your perfect line is the key (just like in this drill). The absolute worst thing is to set up perfectly over the ball and have your brain and your body disagreeing. The body wins every time :) Thanks for this, Alan