Remember that the lightest shadow should be darker than the darkest light. That is a confusing statement.. Basically make sure that no shadow is lighter than any directly lit area.
Shadows receive some reflected light, like the bottom of the left cheek. That reflected light appears to be lighter than the halftone on the right cheek. But, if you take it into Photoshop and use the eyedropper tool, you’ll see that the reflected like (shadow) is in fact darker than the halftone (light).
The value on the left side appears lighter because it is surrounded by very dark values, and the value on the right is surrounded by very light values. The relationships are deceiving, but remember that guideline and you’ll be OK - “The darkest light is lighter than the lightest dark”
For more on Light and Shade check out this lesson: http://www.proko.com/shading-light-and-form-basics/