It's the average humidity that counts so don't panic yet but it shouldn't go below 25% to be on the safe side. Brief spikes won't do any damage though.
How is your incubator set up? Is it in a room that is air conditioned as that will create an environment that is too dry?
It's the surface area of water exposed to the air that creates the humidity so sponges that are half in, half poking out of the well of water can help to increase the humidity. Monitoring your air cells will let you know if you need to increase or decrease your humidity and its very individual for small, home incubators as so much depends on which spot in the house its in.
At this point you could try and keep the humidity up as long as you think your air cells look big enough and try not to disturb the eggs from this point. Make sure your vents are open too as hatching chicks need lots of fresh air.
Here are instructions on how to calibrate your hygrometer to ensure it is reading correctly:
https://www.mypetchicken.com/backya...lp/How-do-I-calibrate-my-hygrometer-H283.aspx
Hopefully you'll get a few cute chicks to expand your flock.