All Great Posts and tons to think about in different locations. I like the baking cake correlation. I am in SE Kansas. I read to keep humidity about 30% and then raise to 70% at lockdown. I have a Styrofoam incubator, with egg turner, still air. The temp does go up and down in it with the ambient temps in the room. I keep a thermometer in it to check. I don't have a hygrometer. I have to add water in the bottom and open small extra air hole in the top of the incubator to increase/decrease humidity. can be challenging. I have had about as good of luck though as with the hens hatching their eggs.
Every year it seems when I was trying to hatch in the house incubator, storm would come and knock our juice off for 3-4 hours, That never turned out well for hatching. Sometimes it would happen when the chicks were a day or two from hatching. NOW we have a back up generator that kicks on if the Electric is off for 30 seconds and runs until Electric comes back on. That is what saved my hatch this time. Elec went off at 10 pm until 2;30 am. But we had power.
Every year it seems when I was trying to hatch in the house incubator, storm would come and knock our juice off for 3-4 hours, That never turned out well for hatching. Sometimes it would happen when the chicks were a day or two from hatching. NOW we have a back up generator that kicks on if the Electric is off for 30 seconds and runs until Electric comes back on. That is what saved my hatch this time. Elec went off at 10 pm until 2;30 am. But we had power.
We have discussed the altitude problem many times . One member had rapid air cell growth and increased the humidity to off set the rapid moisture loss . Just be sure your thermometers are correct . Calibrate them every so often . Eggs will be cloudy before they have visible blood veins . Dark blue eggs are hard to candle sometimes and cloudy is what you look for . So if the eggs appear cloudy on day ten then they started to develop and quit .Sometimes they just quit . Make a mental note of what they looked like at set. You do candle before set don't you ? I do just to make sure I'm not setting cracked eggs . What kind of incubator do you have ? The heating element in LGs and farm innovators and most small incubators is a spring wire incased in ceramic inside a aluminum tube . When the power is shut down the element will continue to radiate heat for a short period of time some as long as four minutes . They can and do malfunction . That doesn't mean yours is. We have known more than one member to have 100% hatch rates in Styrofoam incubators. Still air hand turning . Some eggs are more porous than others and will lose moisture faster . Per set candling will allow for an extra look or two at those . You see there are many things that can affect your hatch rate . But by the by you will figure out what works best for you most of the time . I have a 100% hatch rate . Yep a 100%of my hatches have not hatched at a 100% 
79% has been my best so far unless I lie 
Happy hatching
