I'd place a transparent circle over the head to see if it matches up. If it doesn't look right once you've adjusted the head's outlines then maybe it's the nose or eyes needing adjustment.
ಠ_ಠ
well i feel incredibly stupid. I could have been using this technique to get my heads perfect-ish-ly round long before now.
their heads are basically a perfect circle from what I've gathered, so it makes sense.
I think I may love you.
edit: holy forget this idea has more potential than I thought, I could use this for precise body measurements too! ASAFASFDAF