I've literally been sitting here for the past 2 days reading up on incubation humidity so maybe I can explain simply and save you two days. If I'm wrong I hope somebody will correct me.
You want your eggs to loose a certain amount of weight during incubation. They need to loose 11-13% of their weight on day 18. That controls the size of the air cell.
So how do you control the weight loss? With humidity. High humidity slows down the water loss in the egg and low humidity makes it faster.
If you want to know what humidity to use, you weigh them when you start and weigh them again sometime during the incubation period and if they don't loose as much weight as they are supposed to, you lower the humidity and vice versa.
So one person has to run a different humidity than another one because of environmental factors, incubator type, and apparently egg type. Shoot I had to turn the A/C on the other day and my humidtity dropped because the A/C dries out the air so the air venting in my incubator was drier.
Running dry just means you don't add water, but the humidity inside can still be high or it can be in the perfect spot.
