13 fewer Values, more control
Early on in my learning process I produced lots of painting studies. Each day I’d find an interesting reference image that was full of shapes and contrast, and then try to reproduce it using my art tools. I learned how to create volume with values, but also how easy it is to drift away from the original sketch. If you don’t control your values then you can distort your character’s anatomy.
The face in particular is very delicate. One simple line can change the expression completely, let alone several brushstrokes. So keep it simple and work with just three values. If you feel comfortable you can always add more.