That and I'm too lazy to keep topping it up with water!
First hatch I got 6 from 10 and this one currently has 4 pipped from 6. The reading that I've done suggests 30/40% for days 1-18,then anywhere between 60/75% for days 19-21%.I'm on day 20 and my hygrometer is reading 70% 
You will get a different answer for every different person who posts, lol... really, each person has learned what works for them in their environment and in which type of incubator they use...
The trend I have been seeing is (in general) that if you live in an area with low ambient humidity, then you add some water to the bator... if your area has high ambient humidity, dry seems to work better...
And styro type bators lose humidity more than a hard plastic (like Brinsea) or a cabinet incubator does, so water usually needs to be added more often in those as well...
Personally, I incubate dry... but our humidity matches or exceeds our temps here most of the time, lol...
Sounds within good range...
