You keep the humidity as high as you can without condensation dripping on the eggs.

You'll find a lot of different information because everyone hatches their eggs a little differently.

For me, personally, I will be using press-n-seal on my egg boxes that have no holes drilled in them. Because of that, I'll be venting every 4 or 5 days just to circulate the air a little bit. I also use substrateless method so I will put a lot of water in my egg tubs and sit the eggs on egg crate. Hopefully that keeps my humidity pretty high.