What is the best humidity for incubating chickens?

I'm hatching out baby chicks and i have looked online and everyone is saying all kinds of different % readings from 50% to 65% for the first 18 days and then after 18 days 65% to 75% and for the humidity right now i have finally managed to get it to 55% humidity and when it is time to stop turning them my other incubator i got it up to 80% humidity so what is the ideal humidity for the first 18 days and the last 3 days remaining until hatch?