As of a few years ago the longest horns ever recorded on a Mountain Goat were from a nanny. Never heard of a nanny with enough length of mass to make B&C minimum though. Nor should there be. One of the problems I have with P&Y minimums. Last I knew there were over 45 nannies listed in P&Y none of which exceeded 45".
I'll try to get a hold of Buzzy Cook and ask about why tip to tip and widest width are recorded and not used. He's the most knowledgeable on B&C measuring I know. Must be a reason I just don't have the answer.