I’m still experimenting. There are lots of ok ways to try do this, however few will consistently hit 95% plus - without adding 20+ relays (or more in future).

I’m currently streaming events from 80+ relays, so I can tag which events have been seen by which relays (without actively asking at present).

The current approach is to lookup who a pubkey follows, then get the most recent N events by them, and then process against which relays would be needed to get coverage across the following event set.

It’s got some recursion, so processing speed or pre-processing are considerations vs good enough.

My other approach is to multiplex and perhaps dynamically do this for clients. https://nostrgraph.net/relay

I’ll share updates on Nostr. It’s a little hard to publish a working algorithm since it’s data dependent.