Quite a challenge youve got here, Yvan!If you are really trying to do it photorealistic (tones and details), I believe it would help you to make some adjusting to the reference photo to make it more drawable.
I suggest if youve got Photoshop try adjusting the ref a bit, I would add more contrast to it so the dark side of the face would pop out better, maybe it would be a good idea to add some brightness to the dark side of the face, too.
But if the total tonal balance thing is not an issue to you, just ignore what Iam saying, so far everythings looking good
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