Spend more time looking at what you're drawing, and measuring with your eyes (or ruler, thumb/finger, whatever) and less time worrying about screwing up formulas. The basic canons of the face, understanding of anatomy, and knowing how a skull looks in general are what you need to know. Learn those things, and then expand your knowledge by studying how each facial feature is formed. Then draw each facial feature enough so that you know how to draw it and manipulate it at will.
So step 1) Look at a skull and draw the hell out of it so you understand how the STRUCTURE of the head is formed, and how it looks.
Step 2) Get a few portrait drawing books and study up on what are the typically used proportions (and keep them as a general rule of thumb - you never notice that we all look DIFFERENT despite having all the same facial features? Everything varies!)
Step 3) Draw each facial feature - eyes, nose, mouth, ears, jaw. (The ear is a feature of the head - editted for correctness)
Step 4) Once you can draw them individually - start using them as building blocks for the head. Draw all the features together, as one head.
Reminder: Draw this stuff from real life and photo's as much as you can. You can only accurately change something when you know how to draw it properly to begin with.
Doctors heal you, Artists immortalize you.
"Those who can, do. Those who can't, teach" - bullshit.
The usual staples for anatomy: