/ic/ - Artwork/Critique

I just got to the point in Loomis where he teaches us how to use the ball and plane method for constructing heads. I feel like I may be misunderstanding something though.

He says to drop the middle line down off the ball, then divide the middle line into four equal parts thats size if half of between the brow and the top of the ball. But would that just be splitting everything into quarters then? How far am I supposed to draw the line off the ball?

I'd like to make sure I'm doing it correctly before I dedicate my evening to practicing it.