Great question... I was wondering, too.
For you that keep your humidity so low during hatch... you don't have any shrink wrapping problems???
I have had two hatches now. Both times the first chick to pip took more than 24 hours to hatch and both got a little shrink wrapped and needed help. Other chicks from both hatches pipped and zipped faster and didn't have any issues. I kept lockdown humidity pretty high trying to avoid the shrink wrap (75%). Most people say that higher humidity is useful for lockdown, so I'm really curious about low humidity for lockdown.
Humidity for me without adding water is around 16%. I'm going to try keeping it between 20 and 30% during days 1-18 and see how that goes. I weighed all my eggs, though, so I will adjust too, if they lose too muc weight.