Humidity is complicated. Eggs need to lose a certain amount of moisture before the eggs hatch. If they lose too little or too much the eggs can have trouble hatching. Nature was nice to us in that she provided a fairly wide window that can work, but the nearer to that sweet spot you are the better your chances.
Two people with exactly the same incubator can have different results. There are different factors affecting how much moisture is lost. Things like elevation above sea level or the temperature or humidity of the air in the room can have an effect among other things. The professionals that may hatch 1,000,000 chicks each week in incubators that hold 60,000 eggs each find they have to tweak the humidity if they move an incubator to a different spot in the same incubating room.
Not all eggs are the same. Due to differences in things like porosity or the thickness of the egg whites, different eggs lose moisture at different rates. Eggs stored for different lengths of time before you start incubating have different starting points as they have already lost some humidity. Since the perfect humidity for one egg can be different from the perfect humidity for another, you are looking at the average humidity that gives you the best results for the most eggs. Remember, nature gave a window that works, you don't have to be really precise with the humidity to get good results. The average humidity over the incubation period is what is important, not a short term instantaneous humidity.
My suggestion is to be as consistent as you can with humidity during the hatch and see what results you get, then tweak as necessary. The manufacturer's recommendation for mine was 45%. I followed that as best I could and got a decent hatch, but by opening the unhatched eggs to try to determine what went wrong after a few hatches I eventually found my sweet spot was closer to 40%. The number of chicks that hatched wasn't that different but we try to do the best we can. Out of 28 eggs set maybe 1 or 2 chicks difference at most. Things other than humidity can cause eggs to not hatch, it's not all humidity.
Some people candle their eggs and try to mark the air cell to see how much moisture is being lost, then compare that to how long they have been incubating to see if they need to adjust humidity. Some weight their eggs before they start incubating and weigh them as they go along to see if they need to adjust humidity. If you do this weigh them all at the same time and look at the average weight lost. You obviously need an accurate scale. Since you have already started you can't use the weight method this hatch.
I don't see any reason for you to panic or worry that much, though I know you will still worry no matter what any of us say. Let us know how you do.
