To implement this, all you need to do is download follow lists for each person you follow. It's computationally intensive, but easily cached, so not really a performance problem. As far as dialing in the correct threshold, it should be fairly intuitive for users, although there's lots that could be done to make this tunable/more complex. 

 I meant the literal size of the filter you send to relays. If you follow thousands of people the filter would have 10s and in future maybe 100s of thousands of pubkeys. This feels unsustainable.
Nozzle uses a WoT score too but not for filtering bc of this reason. 

 Ah, right, that's a very good point. Right now Coracle doesn't compile it into a filter, it just drops events. The result of course is that global views will potentially download a lot of events in order to discard them. We need a better solution long-term, as the network grows.