Yes, the egg shell is porous and water evaporates through it. In higher humidity, not as much water evaporates. This can lead to small airsacs and large ducklings which do not have enough room to zip to hatch. In extreme cases you get sticky chicks. In spring, egg shells tend to be thicker (assuming the duck did not lay all winter). This also prevents water from evaporating. So, unless it is extremely dry where you live, I would not add water. I had 17 out of 18 chickens hatch in the last two days. They were incubated in a dry incubator, only adding water when I locked down. I have just had much better success this way.Would 10 - 15% off in the first 25 days affect hatch that much? I meant more that, humidity seems to have a big range that’s considered to be “good.” If their humidity was any higher, I’d be more concerned, unless their humidity is worse than I think??
You do get a lot of different results for what’s “good” humidity, so perhaps I am just misinformed.