What type of incubator do you use?
If it's a hova-bator, if you filled up water wells (one in center, the other in outer edge), humidity would be around 60%-75%.
If it's over 80%, you will see the mist is formed on window(s) of lid.
When I had this issue, I was told to go to walmart and purchase a digital thermometer/hygrometer combo. It's $6 and is GREAT! And after hatching, you can keep it in the brooder box too, or just use it indoor/outdoor for yourself. It's small and has a magnet. We found it in the hardware area. Good luck!
I would listen to Cavendish Chickens and get the hydrometer from Walmart. It makes it really easy and you know exactly when you need to add water or if it's too high.