That sounds like the single-layer form of Net Area Ratio to me. (Documented in the SVRF manual, svrf_ur.pdf.) Does this work for you, or is there something not quite right?
If you need something a bit more customized, there might be some code you can leverage from the Solutions for Physical Verification manual, particularly the examples using DFM. Look for calbr_solns_pv_user.pdf in your mgcdocs directory.