Yes, that's right about the nut being at about eye level. It's a handy way of checking for the right position.

MikeL2 - You said "However I managed to devise a kind of "hybrid" style in that I held the neck with my thumb wrapped around it but when I wanted to use barre chords I learned to slide my thumb into almost the classical position."

That's not a hybrid position. It's exactly what everyone does if they normally play open string chords with the thumb wrapped around the side of the neck. The nature of a bar chord forces you to put your thumb behind the neck in the proper (classical) position. It's virtually impossible to do a bar chord any other way as far as I know. What you are describing in your "hybrid" style is exactly the way I used to play and what I am now training myself not to do.