1. When drawing the line indicating the oval for the side -- does the line follow parallel to the center-line, or am I meant to simply guess? I've been drawing it keeping it parallel to the central curve.
Not sure what u meant, but it's like a ball been cut parallel to the center(of-course u decide the center)
2. The divisions of the face aren't equal at an angle, the nearer plane is wider. How do you determine the widths?
you can study perspective and it will help you
but I mostly recommend u draw a-lot of portraits from life/pics, and you will naturally see it.
it's not mathematical measuring
3. How do I determine the widths where the chin connects?
again, no math, as you keep drawing you will see it. it is sorta like you see 3d, on the paper