By default the server uses ports 27015-27017. Each server uses 3 consecutive ports. The first port is called the base port. Normal game traffic uses the middle one of the three eg. 27016 by default.
When there are multiple servers running on same computer, other servers are configured to use different base ports. The values FM servers currently use are 27015, 27025, 27035, 27045. This is however up to the server admin.
Maybe you could just prioritize the whole range 27015 - 27047.