Humidity in an incubator is a very important issue - as it controls many things but what you need to do is to enable the chicks are the right size to escape the egg. The biggest and I mean biggest cause of eggs hatching in your incubator is humidity that is too high. Humidity for chicken eggs is 40-45% for 19 days and 60-65% on the last 2 days. If you have a still air incubator this is fine. Who still has a still air incubator? Not many so we mostly have moving air incubators (ones with fans in to ensure even temperaturs in the incubator). The problem with this is it gives false readings on hygrometers you may be using. None of the current incubation incubation books I have read have understood or explained this point and hence the incubators improve but hatching does not. I have however calculated humidity readings from the current hygrometer options and to get best hatchings I suggest. Still air incubators 40-45% for 19 days then 60-65% for next 2 days including hatching day. Moving air incubators (fan assisted) 28-35% for 19 days then 48-55% for next 2 days including hatching day.  Why is it different? Really it is not it is just the moving air is affecting the readings on the hygrometer. (Please not RCOM incubators are accurate moving air incubators and can be run at 40-45% and 60-65% as their set up reads where the air is not affecting the probe)