The best I can provide is based on when I was setting up database mirroring. First, iirc, you need to select a port number above 1024, and I thought under 32768. When selecting this port number, work with your network services personnel to help identify an open port or range of open ports you can use such that they can document the port number(s) such that future applicatioins don't suddenly take the port(s) you and Network Services assigned.