What's your incubator humidity level days 1-18 then 18-21?

Your humidity is not too low, I aim for about 40 - 45% and then for the last 3 day, 65%.  And I usually have great hatch rates, most being around 95%.

FYI--  Everyone needs to find a humidity the works in their own specific environment and incubator. TIme of year effects my humidity levels--humid summer weather brought a drop in hatching%. Because I was not looking at the eggs, the aircells actually.

Development of the air cell to the proper size is the key. Humidity is managed to that goal. Do you have a diagram to compare to while candling?

Search  on the 'net using key words: air cells day 7,14, and 18 and you will find a number of diagrams. Pick one that is easy to print and understand for you. Some are black and white, others are colored.. Use it while candling. Hold the egg near it and compare. 

For candling--use any small LED flashlight. If you can see the veins, then that flash light is more than good enough to see the air cell development.

Adjust the humidity to make the air cell grow faster, or to slow down development. THere is an optimal moisture loss for eggs; too much or too little , it won't hatch. 

By far, the biggest problem is not enough air cell development. You can stop the drying of the eggs but you can not speed up the loss of moisture very well once you are headed into lockdown.

ANy questions? Let me know . . .

I aim to keep mine between 30-45 day 1-18, my incubator does this on it's own as long as the built in water rings are filled.

After day18, I throw in a warm, damp sponge and my humidity jumps to 70-75 [water rings filled and sponge combination]

My hatch room maintains a steady temperature of 72-75, I keep the air vents closed in the room so only the return register is running. I have a thermometer/hygrometer in the room and the room itself stays between 50-60% humidity on it's own. Closing the registers has really helped me keep my temps from too much variation. I also keep the door closed in that room [it's my craft/chicken room] so there isn't a draft.

As the PP said, it varies for everyone, mostly you'll just learn from trial and error...at least that's what I did.