I guess not every relay has the stable flag, would be curious to know what the general percentage is
You can check with [Tor Metrics][] how many relays have the Running flag and how many have the Stable flag. By looking at the graph I would estimate around 80% of Running relays have the Stable flag right now.
[tor metrics]: https://metrics.torproject.org/relayflags.html?start=2018-01-22&end=2018...
-- 4096R/A83CE748 Valters Jansons