First that 21 day thing is just a guideline. It’s not that unusual for chicks to hatch a day or two early or late, whether in an incubator or under a broody hen. There are a lot of different things that can make them early or late, heredity, humidity, how long or just how the eggs are stored before incubation starts, possibly size of eggs, and just basic differences in the eggs. A really big one, especially in an incubator, is average incubation temperature. If the average (not instantaneous but average) temperature is a bit high they can be early, low then they will probably be late. So look at the 21 days as hitting the bull’s eye. But just hitting the target still gives you a score.
There is a common mistake people make, are you counting the days correctly. An egg does not have a full day’s incubation the instant you put it in the incubator, it takes 24 hours for an egg to have a day’s worth of incubation. An easy check on your counting is the day of the week you start them is the day of the week the 21 days is up. If you start them on a Tuesday the 21 days are up on a Tuesday. You might be surprised at how often this mistake is made. But even with lockdown and hatch date a full day off most people still get great hatches. Nature has a lot of forgiveness in that 21 day thing.
Before the chick hatches it absorbs the yolk. This allows the chick to live off of that yolk for three days or more without eating or drinking. That’s why chicks can be mailed. Sometimes my hatches, incubator or broody hen, or over within 24 hours of the first one hatching. I’ve had broody and incubator hatches drag out for almost three full 24 hour days and that is with all the eggs started at the same time. That’s why the chick absorb the yolk, so the first hatchers can wait on the late hatchers. So you can leave the chicks in the incubator for three full days before taking them out. I’ve had broody hens take over four days to bring the first chicks to hatch off the nest and they did fine, but the broody is talking to her chicks. She knows they are not in trouble. But since I don’t speak their language I consider 3 full days my limit in an incubator.
You will often see warnings about opening the incubator after lockdown. It is possible to shrink-wrap a chick if the humidity drops too low after a chick has pipped but before it has finished hatching. That’s where the membrane surrounding he chick dries out and shrinks around the chick, preventing it from moving to hatch. This can possibly happen, I’ve done it. But the reality is that it very seldom happens. There are plenty of people on this forum that ignore this warning and open the incubator during lockdown all the time. I personally will open the incubator to take care of an immediate problem, realizing there is some risk but that the actual risk is pretty low. I consider it good practice to not open the incubator unless you have a good reason. But if I have a good reason I don’t hesitate.
So you can leave them in there for three days without a problem. Or you can take them out as they dry off but realize there is a small risk involved.
Good luck with the hatch.
OK, so what if the incubator goes over the 70 for humidity? I feel like I cannot get it under like 74%? 
