Symmetry and proportion are essential elements in the structure of a portrait. One of the best tools for achieving symmetry and proportion is the box. Creating a box whose dimensions roughly approximate the size of the head and its movement in space, will go a long way to creating a strong portrait. John Singer Sargent’s work is full of excellent examples of using the box.
After drawing the box in space, find your center line for the front plane.
Divide your box into thirds. The middle third will represent the distance between the brow ridge and the bottom of the nose. Use this same distance to adjust the line at the bottom of the chin and the hairline or top of the head.
The width of half of the box is the same distance you just used (brow ridge to bottom of nose).
Halfway between the brow ridge and the bottom of the nose is roughly the bottom of the eye socket. Halfway between the bottom of the nose and the chin is the bottom of the lower lip.
Once you’ve found these proportions and marked them, hold to them, and continue checking them as you draw and paint. Keep in mind, you are not measuring the model, you’re constructing a box and dividing it into the proportions of the human face.
If something seems off, check those proportions.