Without any water added, my RH will get down to 16%. I like to keep it around 25 to 35% for days 1-18 and then up to 50% for hatching. It will go up as hatching begins. At least once per season I miss an egg marked to go into the hatcher and it hatches in the incubator turner with a RH around 35% so I don't worry about getting it above 50% to hatch. What you want to do is keep an eye on the air cell size. The air cell size will tell you what range you need to be at for the whole process for your incubator and environment. If the air cell is too small, for example, the chick will be too big to turn properly to pip/hatch.