3d z-order problem

We created a site using some 3D Flash techniques at http://www.harmonycollection.com/. However, we are looking for alternative ways of determining a z-order for objects in the 3D view. Currently, my system simply determines the distance of the centerpoint of each object in plan vs the upper left corner of the screen. This system is not foolproof (ie. objects at 45 degrees to one another sometimes are in reverse order). Does anyone know of any websites that discuss methods of making these calculations more accurate? Any help would be greatly appreciated.

fredster