Thank you for the offer but those parts are done. A parametric center would be very complicated. Every piece in the z axis changes and most of the xyz dimensions change.
I know the information is spread over these forums, facebook, G+, and thingiverse, but I am almost done. I have the the corners, middle, and z axis done. I am trying to re-design the feet and rollers before release. If I release it now and then new parts shortly after I will have to deal with all the emails from upset people. This last 525 update sent a flood of questions, and mad customers my way. I is just best if I release this when it complete and make the transition as smooth as possible.
Sorry for the wait.