A lot of the guidelines you read on here come from the commercial hatching industry, the people where one hatchery might hatch 1,000,000 chicks a week using incubators that hold 60,000 or even 120,0000 eggs each. Just a minor change in hatch rate can mean a whole lot of chicks over the course of a year for them. I normally hatch around 40 chicks a year, half of them under broody hens. I would not notice a 1% difference in hatch rate a year, but for that one commercial hatchery 1% would mean 500,000 chicks a year. That’s noticeable. The guidelines are important, you need to know what your target should be, but there is a lot of leeway in many of them. There is not a sudden change from where all eggs will hatch to where no egg will hatch. The further you stray from the ideal conditions the less likely the egg is to hatch, but with many of those guidelines you have to travel a pretty good distance away from the ideal before it gets very noticeable. Know what you are shooting for, but just do the best you reasonably can and you will probably do OK.
Not every incubator is the same. The commercial boys know they have to learn each new incubator and tweak it to get the best hatch rate even when they have identical models. They also know that if they move an incubator from one side of the room to the other, they will have to relearn how to hatch in it. Conditions have changed. It may be the temperature or humidity of the air going in has changed, it may be something else.
Not all eggs are the same. You get the obvious differences in how and how long they were stored or maybe they were handled differently. There are also differences in each egg. Some have thicker shells, more porous shells, maybe the egg white is thicker or thinner. That means the same humidity is not perfect for each and every egg. What you are trying to do with the humidity is to get to a certain moisture loss in the egg. Since each egg is different you would need a separate incubator for each egg and carefully tuned to that egg to get it perfect. That’s not going to happen.
Last year a brave soul did a hatching journal on here to keep track of moisture loss. He carefully weighed each egg to record weight loss, thinking to adjust the humidity to get it right. The weight loss per egg was way different. There was no way he could hit the perfect humidity for each egg. The only way to do that was to take an average of all the eggs and try to get it close for as many as he could.
The good news out of all this is that there is a pretty wide band of humidity where the eggs have a good chance of hatching. You don’t have to get the humidity perfect for each egg. As long as you are close you can be pretty successful. You need to determine what humidity works for you and that comes with trial and error.
My suggestion is to do the best you reasonably can, select a target humidity and try to stick with it. After the hatch is over open any unhatched eggs and try to determine why it did not hatch. That’s not real easy, by the way. There are a huge number of reasons an egg may not have hatched. Just do the best you can then try tweaking the incubator to see if you can improve on your hatch rate.
For what it is worth, the commercial operations hatch about 90% of the eggs they put in the incubator. About half their failures are due to something that happened before the egg went in the incubator; fertility, how the egg was handled, health and nutrition of the parents, dirty eggs, things like that. About half the failures are due to the incubation process; heat, humidity, improper turning, not good fresh air exchange, something like that. With them hatching 1,000,000 a week their averages mean something. With the number of chicks I hatch my average doesn’t mean a lot. I’ve had 100% hatches and I had one that was only 33% due to me improperly transporting the eggs. I sure learned something from that incubation.
Good luck. Lots of us make mistakes and a few of us actually earn from them.
