To draw your Manga character's face proportions correctly, first you need some guidelines. Here's how:
Find the half way point on the vertical guideline, and draw a horizontal guideline across the width of the head. This is the eye line.
Half way between the eye line and the chin, draw another horizontal line. This new line will indicate where the bottom of the nose should go.
Half way between this nose line and the chin, draw a short horizontal line. This line is the shadow below the lower lip. You draw this first because it can be difficult to draw the lip line in the proper location without much experience. When you get more comfortable with the face, you can cut straight to the lips.