Humidity does matter during incubation. The egg needs to lose a certain amount of moisture or the chick cannot hatch. If it loses too little moisture the air cell is too small so it can suffocate after internal pip. It just doesn’t have enough air to breathe when it’s switching from living in a liquid environment to breathing air. If they do manage to hatch they are often large and mushy, not real healthy. If they lose too much moisture the membrane around them can shrink and imprison them so they cannot move to hatch.
The good news is that there is a pretty wide range of humidity that works, but there has to be. There is no one perfect humidity for each and every egg ever laid. Different eggs are different. Some eggs have more porosity than others. Some have thicker whites. This affects how fast they lose moisture. Some eggs are stored longer than others before they go in the incubator. They lose moisture during storage, how much depending on how long they are stored and under what conditions. Eggs stored a bit don’t need to lose as much moisture in the incubator as fresher eggs.
Another problem in determining the right humidity is that there can be a lot going on inside the incubator. It’s not just about the humidity in the incubator. How high you are above sea level will determine your air pressure. That can have an effect on how much moisture an egg loses. The temperature and moisture content of the air entering the incubator, plus the volume of that air, has an effect on how much moisture the egg loses. Even how your incubator is vented can have an effect. Still air versus forced air has an effect. It’s a darn good thing there is a pretty wide margin of moisture loss that will work. Otherwise out hatch rates would really be consistently lousy.
I can’t tell you what humidity will work best for you. We all have different answers to that and what works best for me might not work that great for you. What I suggest is to pick a humidity and maintain it as best you can. Then after the hatch open the unhatched eggs and see if you can tell by looking at the unhatched chicks if you think the humidity was too high or too low. There is some trial and error involved.
Commercial hatcheries that might hatch 1,000,000 chicks a week using incubators that hold 60,000 or even 120,000 eggs each have to go through this trial and error process to get an optimum hatch rate. If they move an incubator to a different part of the same incubation room the best humidity level may change. They examine the unhatched chicks and make adjustments based on what they see.
Good luck with it. Sometimes there is more art than science in getting a good hatch.