A little help with eight Norstar CICS

A friend of mine owns a small business that has a Norstar compact ICS. The gentleman that used to maintain it for him passed away recently. He needs to add one more drop and extension to the system. Any advice on where to start? There were no notes in any of the manuals. The system is currently set up with 15 sets.

I maintained a Merlin system many years ago, but nothing since.

I do not get tone on what would be the next pair on the demark block,