1. Units of one eye
Usually the space between the eyes is the equivalent of one eye and the space between the outside corner of the eye and the edge of the front side of the cranium is half an eye
I also drew two eyes between the bottom of the eyes and the corner of the lips but this came up while I was making that hand-out so I am not too sure if it a generic proportion.
2. The triangle
The shape of that triangle would vary from person to person so it is a great way to check the likeness. On this model the distance between the bottom of the nose and the pupil is equidistant
3. Features line up
The inner corners of the eyes usually line up with the wings of the nose and the inner edge of the Iris lines up with the corner of the mouth. It can be the pupil on a male model.
