Artists write about painting portraits….

The distance from the chin to the nostrils makes up a third of the face. It is the same from the nostrils to the eyebrows, and again from the eyebrows to the hairline. From one ear to the other, the distance is the same as from the eyebrows to the chin. The width of the mouth is the same as that from the middle of the lips to the chin.
Leonardo da Vinci