That picture and this painting of her face share the exact same principle as in this Georgetown Atelier Tutorial, which was really fun to discover.
The key is that the darkest dark is in the cast shadow. In the sphere it is the shadow right under the sphere. The core shadow is second in degree of darkness and lastly is the dark side filled with reflected light. The light on the model’s face was exactly the same. If you examine the picture bellow, look in the groove of the chin, under the top lip, right under the nose, and right under her eye in the dark side: Those are all the darkest dark. Second in degree is the core shadow, which is slightly less dark (like on her cheek), and the reflected light, which is significantly lighter here than the other shadow areas.
Seeing this principle clearly allowed me to paint with greater clarity, and better yet, I will be looking for the same principle next time I paint and every time I paint.