You are correct. Hatcheries are often 90% correct with vent sexing, which is about as good as it gets with chicks that are not autosexing or sexlinked.i saw someone advertise they sexed their chicks. It's just a "regular Joe". Is that possible? i thought "sexers" were few and far between. and they breeds weren't auto-sexing at all. How often do you hear of backyard hobbyists that can sex chicks?
I would not expect much better than 50 with day old chicks using most of the other methods. Some breeds you can tell at three days old with feather sexing but not 90%. After a week or so you can tell with some breeds by the length of tail feather. A bit later you can often tell by comb and wattle size and color.
