looking good bro!
would you be interested to know that Leonardo Da Vinci found a formula for the skull? along with the whole human figure. Michaelangelo and other artists have similiar formulas as well.
It deals with a box and lots of triangles. It's a very interesting formula that will let you always make a very proportionate skull... for the entire skeletal system.
It progresses into more and more covering the human figure and the methods used there. I found it very informative.
This guy Riven Pheonix has some good material he teaches. Some like him, some dont. He's boring to listen to, fascinating to watch and learn from. I have used one of the Vilppu manuals and it simply didnt stick in my mind as this does.
if you have 22 minutes check this out, it's enlightening and just a start