the humidity affects how quickly the egg evaporates or loses water weight. so with lower humidity conditions (dry hatching), the eggs own evaporation is contributing to the incubator humidity. if your incubator is pretty full, it should regulate itself well (depending on your house (and region) humidity). I find that if I only set 6-8 eggs in my incubator that holds 40 eggs, the eggs don't regulate as well (10-15%) and in that case I add a small sized wet sponge, cut up about the size of a quarter. another factor is still air vs forced air. a forced air incubator has a fan kit and seems to evaporate the sponge very quickly. a still air will maybe run slightly lower humidity because there isn't a fan moving air on the sponge.
25% is the lowest humidity I allow and 45% is the highest, but I don't usually run it that high.
I try to aim for about 30% at first. around day 16ish I might increase it to 40%, this depends on their weight (I weigh on day 0, 7, 14, 18 to check proper weight loss - goal is 13% loss). so if on day 14 they have lost plenty of water weight and are close to goal, then I might bump it to 40% to slow down the weight loss before lockdown happens. you can also look at the size of the air cell to judge weight loss.
I candle each day and look for an internal pip..it should happen on day 19. you can increase the humidity on day 19 if you want. I like to wait until the internal pip happens, then humidity is raised to 55-65% and I do not open the incubator anymore (lockdown). when a chick hatches, it will cause a humidity spike up around 70-80% and that is fine and expected!
