Trying to capture a likeness in a painting is HARD. If you add a little too much shadow in the wrong spot on the nose for example, it totally changes the shape.
To help myself out, I took the reference image, posterized it in Photoshop then made a map of the tonal values in the drawing. It really helped, though I found it took some of the fun out letting things flow naturally while painting. I feel it's my best likeness yet though! Painted with India ink.