Rotation point for parent track motion

Any help is appreciated. Using Vegas Pro 10.

My project is 720x480 and I bring in 2 photos (rectangular shapes with different pixel dimensions) and pan/crop and place them on separate tracks. At this point, the heights become the same (stretched to fill frame), and the widths are close enough.

Using 3D track motion for each track and a parent track, I am able to position the photos at right angles to each other (like a front and left side of a cube). My objective is to use the parent track to simply rotate this 2 sided "cube" so that the left side comes into view and replaces the front photo which moves out of view to the right side during the rotation. Nothing complicated, but a nice effect.

My difficulty is how to figure and set the rotation axis point so that the left side moves into the same exact position that the front photo occupied. I can get the photos to rotate, but the hinge point (axis) for the "cube" is way off. I can't seem to figure the photo dimensions after pan crop w/stretch to try to compute where the center should be.

Maybe I'm just too close to it. Hope I explained this well enough. Thanks for any help.