Recommended setting for `trust proxy` on express

What are the recommended settings for trust proxy when using Express on node? None of these seem to work: 'loopback', 'linklocal', 'uniquelocal' as the proxy’s address is not in that range. A value of 2 seems to work for me now, but I am wondering if there is a better setting for a subnet that will reliably filter out the load balancers / proxies that fly.io uses.