I like to keep it simple when it comes to hatching and incubating eggs. Starting with a good thermometer, running it at the correct temperature, and within the correct range for humidity. Doing that, I have been very successful.
I realize that people like to try different things to tease out an extra couple percent, and I am not disputing those efforts. I also realize that it requires hatching thousands under precisely controlled conditions to really know what works and what does not. Obviously, some here have enough experience to know what works for them, and not.
If my incubator runs at 99.5 degrees (forced air), and the humidity runs between 40-50% humidity, then in the range of 60% during the hatch, I have very good hatches. In the range of 90-95% (of fertile eggs). That is if I store the eggs well, and for less than 7 days. I tried storing eggs up to 14 days, but for me, that is too much. I would not be beyond storing them for up to ten days.
My point is not to the more experienced. It is to the less experienced. Make sure the breeding birds are in good condition, control the temperature and humidity, turn the eggs as they should be turned, keep the incubators/hatchers clean, store the eggs well, set only clean eggs, and you will do well. If that does not produce good results, then the birds are likely the problem.
There is no substitute for a good incubator.
Eggs tolerate a small range in conditions, and still hatch. They are tougher than we give them credit for being. I accidentally had my incubator turned off for 16-17 hours this year. Two batches a week a part still hatched at plus 90%. I think it was near 69 of 73. The only affect was a delayed hatch, and a hatch that drug out longer than normal. They tolerate dips in temperature, much better than spikes in temperature.
For those starting out, buy a GOOD incubator, stick to the keys, and keep it simple. Once you experience sustained success for a length of time, then you can tweak it as you please. Then there is no wondering why there is a change in results. Good or bad.
Ronnot1 is the resident incubating and hatching expert. He has good advice.