Lou,
This sounds like it might not be something you want to track in Mocha. I'd look at using the normal After Effects camera tracker that got bundled in last version. You'll be able to grab a null from that will be in the right 3D space with a camera that's pretty close to right. Mocha is great but this sounds like something where you want have a better idea about the volume of the space and not just one tracked point.
Brian
Brian Behm
Rooster Teeth Productions
Austin, TX