Cabinet Incubators and Humidity question

[Noah Way Farm]

In my dreams, I will soon purchase a cabinet incubator, but have a question for those who own one. The GQF advises you can have a "continual hatch" by starting a new tray of eggs weekly, and rotating the oldest eggs into the brooder box on the lowest shelf. I just wonder about the humidity level for those eggs vs the newer eggs up above. How is this controlled?

 

[FortCluck]

I'm interested in this answer too because I plan on making my own.