Slasher:
We don't operate to FAA certs around here aterp, but is there
anywhere I can obtain that AC just for reference?
Advisory circulars of this type provide guidance and, as such, are not regulatory. I suspect performance engineers in many places other than FAA-dom are using AC 120-91.
Another example is FAA Order 8260.52 that provides criteria for RNP AR IAPs. It is being widely used outside the U.S.