The shape should be more along the lines of a human skull, at least judging by the images i found online, what you built has more of a ball shape.
Here's a good way of getting proportions closer:
Load a .glm or .xsi from raven and build your mesh around the head, line up the features such as the eyes (visor) and mouth so you will know they are correct.
I also suggest you dont get too caught up in keeping your mesh all quads, tris will define the end product anyway. Don't be afraid to build the whole thing in "modules" and connect everything together in the end.
Keep the updates comin.