How to Draw (Robertson's book) study thread

[SamPeterson]

I'm a bit stuck with one of the concepts if anyone can help me. On page 74 how are you supposed to know where to correctly place the minor axis without knowing where the perfect center of the ellipse or square is ahead of time? It doesn't seem like there is enough information to correctly plant the minor axis. It's possible to find the center vertically by making any size rectangle and finding the center of that, but that doesn't show you the horizontal placement. If the minor axis is placed incorrectly it'd make a warped circle that isn't proportioned correctly, and wouldn't result in a proper cube. And without knowing where to plant the minor axis I can't do the exercise or the following ones that require you to know how to create a cube using ellipses. I hope that makes sense.

Thanks.