For the best information on humidity, go to the Learning Center, click on the How to Pages. Read the article by Bill Worrell on dry incubation. Many of us new comers to chickens have been useing this medthod. Only Bill has put it in words that everyone can understand.
More chicks have been drown in the egg than have been hatched at high humidity.
As a child I can remember my grandfather always telling us to keep the humidity down in the incubator. This was a walk in incubator that could hatch over 15,000 eggs at a time. I used this incubator for a number of years after I got out of the Navy in 1962. I believe he built it in 1935. there was a number of chages made to it over the years. But it was working when I leaft N. J. in 1971.
Keep your humidity down and you will hatch many more chicks.
Think of it this way, how does a hen raise her humidity to 50 or 60%. Even during the last three days the humidity in the nest doesn't go up that much. Yes, it does some, but that is because of the chicks breaking out of the egg.