I was advised to keep humidity levels quite high during incubation around 50-60% and then during the hatching stage around 70-80%. Particularly so for Guinea fowl which seem to have relatively difficult time hatching and may get stuck to the membrane and die once they have made a hole and moisture levels start to drop inide the shell. I was also told that for Geese and Muscovy ducks you need to spray the eggs during the latter stages. Much depends on what type of incubator you have, but the guidance notes are usually pretty good. My Brisea Octagon 20 advance has 2 slots for water and they suggest to get the humidity higher put cardboard bent over into the trays to increase evaporation and increse the humidity. It also depends on where in the world you are. In Malaysia (here) the humidity starting level is already pretty high (unless you live in a controlled environment with aircon etc - I don't).
Hope this helps!