I have been into chickens since childhood and am now in my 50's. I have raised chickens also for the giant guru companies of this nation. I now only do BYC. so I say for forced air 35 - 40 percent humidity up to day 18, then raise to 65 - 70 percent humidity, lock down til hatch. By lock down I mean lock down, you know don't open it. Lock down means nothing in and nothing out. So don't open it to put in more water after day 18, and yes the humidity at 65 - 70 percent, may go down. No big deal leave it alone!
Just a note: If your percentages are 5% lower than the above mentioned humidity percentages, for me that is no big deal. However, I do not recommend going higher than the percentages above, your chicks may drown in the egg at hatching time if you go higher than the above humidity percentages.
hytop