As somewhere by the river suggested, increase surface area, so bigger bowl.
Also, are you measuring humidity at egg height? -I don't have a fan, and have found that humidity changes a lot between measuring on top of set eggs, or measuring at the level they're sitting on. I would be inclined to do a trial run and weigh eggs rather than trying to get it spot on now.
Again, if you don't have a fan, could you have the/another water trough higher in machine? I've thought in the past that if I fixed a little gutter/trough to the inside of the hood, near the element, where it's hottest and filled with water, then more water could be evaporated than is currently possible with water pans under eggs, at very base of machine where the air is coolest.