Knees in front orthograhic view reference image lower than in right side view
You're absolutely right theluthier in pointing out that the 3D artwork should be the main reference. We've discussed it already earlier here on CGCookie (for example in the context of a robot character here) that front and side views often don't match exactly as it is the case here too where the knees of Piero in orthographic front view are significantly lower than in side view which served as the positioning of the knees along the local/global Z axis. But the real beauty of the model comes from the colored 3D artwork.
PS: Coming back to a model after a certain time also helps. My second try with Piero looks way better! Posts coming soon.