I agree with this as well I keep my incubator between 28% and 35% until day 18 and then I go no more than 50%.
The reason why I do this is because I had another hatch before this one where the humidity spiked after day 18 because of the external pips and it cause condensation in my incubator. I lost 9 fully developed checks that hatch.
The hatch that I just recently did everything went perfectly. I had a 90% hatch rate.
The humidity all depends on your location, if you are in a humid area you could run your incubator dry with no water. I live in a very humid area and I would not add a lot of water because it just causes a lot of problems. You will have small air cells and that can cause the chicks to drown (what happened to me last hatch).
I have learned that incubating is basically trial and error. You are not going to know what works for you unless you get that error.
@muddy75 and
@Kiki are really good at incubating. They both helped me get that 90% hatch rate this time.