I had humidity down around 66-68% and lost a chick who's membrane was tough ad dry and papery. I have the humidity up to 72% since yesterday when that chick started having trouble, now I have two more pip holes and I can clearly see a difference in the membrane. It is soft and felxible and the chicks are chirping and zipping.
This is my first hatch and there are others who know way better than I, but that's my meager experience. Good luck to you.