I agree. I keep mind around 50% with chicken eggs, regardless of day. But then I also have a rotating incubation schedule. Eggs get laid on day 1, they go in right away. Eggs get laid on day 2, they go in on Day 2. Eggs get laid on Day 3, they go in on day 3. On day 21, the first eggs will hatch, and whatever eggs get laid that day will be put in the incubator. So I am adding eggs to the incubator every day, and I have hatches almost EVERY day as well. There is no "day 17-21" for me, because every egg is on a separate schedule. So I don't try to raise the humidity at the end.
But then again, a mother hen can't exactly sweat a lot heavier at the end of sitting, either. So it almost seems pointless to raise the humidity. HOWEVER, the hatching eggs themselves will help raise humidity as the liquid on the inside becomes exposed and gets evaporated into the air inside of the incubator. But that first chick to hatch under a momma hen still has to deal with the same humidity she had on day 4.
Now during the winter, the air here gets EXTREMELY dry, and I have to either add more water sources (sponges, shallow dishes) or mist it at certain times of the day with a spray bottle. But I still don't try to go too far above 50%.
I fully agree with you about the temperature and humidity staying constant under the hen. What the hen does not have is a poxy fan incessantly blowing out of her lower bits and onto the eggs, drying them out

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