I recently took a 1-day workshop on drawing portraits. I wanted to learn the instructor's technique of blending watercolor pencils and acrylic paint.
The teacher began by reviewing facial proportions: The head (an egg shape) has eyes and ears half way down the head. Another half (or three-quarters of the way down from the top of the head, the nose ends. And, halfway between the tip of the nose and the bottom of the chin is the mouth.
It's all very systematic. And the space between the eyes is the width of one eye. In class we used a ruler to measure everything out.
It's essential to keep in mind when drawing or painting a portrait...
(Though I've discovered it's a little more challenging when the face isn't staring straight at you!)