I think most people try to keep the humidity at least 35% for the first 18 days. Personally I keep mine around 40% to 50%. Right now even with all of the vents open it is 45%. When I set my eggs in lockdown on day 18 then I raise the humidity around 65% to 70%. If it goes a little higher, I don't worry.
Differs among people. I used to think 35% was way too low, then after many seriously horrible hatch-rates (lots of dead, unborn chicks) I realized my humidity was much too high, dropped it to 35% through most of the incubation process, and now I get 100% hatch rate every time.
I keep mine between 45-50% and then raise it on day 18 to 65% but it really depends on the person. but I definately would not go below 25 that seems to low