Mine do best with 45% through incubation, and 55-60% for the last three days. I just hatched some that finished late last night. The weather's cooled, ambient humidity has dropped. Part of the incubation was don to 30-35%, then for the last 3 days, I had trouble getting above 50% even when the eggs started to hatch. The last two chicks were taking a long time to zip, got shrink-wrapped, and I had to help them out.
I was surprised what a difference just that slight drop in humidity made.
I wonder if that's my issue. This is the first hatch with the furnace on. The water level was fine for the summer but I had to add dishes of water to the incubator to get it up there. Hopefully they do fine. It's my last hatch before winter. I was going to stop but I got a hold of 3 dozen dark layer eggs. I couldn't NOT incubate them!