I've tried using the product twice. It just does not work well.

First of all, the edge is straight, which seems like it should be, except for the fact that my neck and the back of my head is not a perfect cylinder. Applying a straight edge against a rounded surface yields edges that curve upward so I don't get a straight line.

Second the elastic band just doesn't work well. I could find no angle or tightness that worked at holding the guide firmly in place. Yes, I know you are supposed to hold it with one hand, but you're just a quick slip or a slow slide away from disaster.

Third, it's just not very durable. The gummy plastic of which this item is comprised has already torn where the elastic threads through it. Which brings me to my next point.

Fourth, the material is also slick. It's not rubbery, there is no significant texture to allow it to grip your head and stay in place. It slides easily with very little pressure. When you're using something as a guide, the very last thing you want it to do is slide around on you.

Lastly, even if everything above were not true, at the end of the day you still cannot see the back of your own head well without an array of mirrors and there is very little you can do to be sure the thing is level when you're using it.

As the title reads, I had better luck just using tape or closing my eyes and feeling my way through it without a guide.