My initial thought to locate this subassembly was to create a saved analysis
measurement in the top assembly between the two faces, and then assemble the
subassembly using the plane in the subassembly offset from one of the two
faces. Once this has been done, you could create an assembly level
relationship so that the distance that the assembly is offset from the face
was half of the distance of the saved analysis feature.
Just something to ponder.
Scott Schultz
Principal Consultant
3D Relief Inc.