Most people will go by the book and the book may tell you to 65% the first 18 days, and 80% the last 3 days. I usually do 30% the first 18 days, and 70% the last 3 days. To be safe, I would advise you to keep your humidity at 40% the first 18 days. It has been proven by many of us that if you incubate the first 18 days at 55-65% humidity your hatch rates will be lower, because the egg will not lose enough of its liquid weight the first 18 days, and by day 20 when it has started to tear through the membrane, the chick will get a gush of water in its nostrils and drown. Usually that's the case of what happens when the egg is cracked open, and the chick is completely formed.