I have never hatched duck eggs. From what ive read they need higher humidity then chicken eggs. So even if it was to low for duck eggs doesn't mean its to low for chicken eggs. If humidity jumps to high try removing the other plug instead of water. Keep an eye on temp though. You might have to...
If you have enough room in there try using the short mason jars with a half a sponge in it. If humidity goes to high remove the sponge. If you still have to much try a shot glass. Make sure you're giving it time to stabilize before you make another change.