To ensure we make our measurements halfway across the world correctly, we require the subject to measure the length of their mouth (from mouth corner to corner) using a straight ruler. This measurement, once provided to us will allow us to scale the frontal profile image correctly, regardless of the distance from the camera.
Similarly for the side profile image, we require the subject to measure their philtrum length. This is from the tip of the upper lip to the ‘subnasal’ which is where the nose septum just begins.
If you find method 1 difficult, you can also try method 2