Did you see the air cell when you candled? It's just a pocket of air in the large eng of the egg. That is what the humidity levels affect. The air cell is very important when it comes to hatching, if it's too small, chicks will often drown, have heart attacks, and a number of other issues. If they are some how able to hatch, they are at risk for mushy chick disease. If air cells are too large, chicks have trouble hatching and often can't move into hatching position. So air cells are very important.
To get the correct air cell size, you need to have the correct humidity through out incubation. There are 2 ways to determine this. You can compare air cell sizes to a chart (not as accurate, but better than nothing) or you can weigh the eggs before incubation and make sure that they lose the correct percentage of mass before they hatch. When comparing to a chart, if the air cells are too large, you need to increase your humidity. If the air cells are too small, you need to lower the humidity. When weighing, if eggs are losing moisture too quickly, increase your humidity. If eggs are losing moisture too slowly, decrease your humidity. I can send you a few articles and diagrams on this if you'd like.
Correct humidity all depends and where you live, the current season, the humidity outside of the incubator, how porous the eggs are and many other factors. Because of this, you really can't have standard numbers; they works for some, but can be disaterous for others.
