Regulate hygrometry to promote hatching

How to promote hatching thanks to the humidity level? Recommended hygrometry, presentation of the necessary equipment, advice and method.

The problem of eggs that do not hatch, despite being fertilized, encountered by breeders gave me the idea of this topic.
The importance of the humidity level during egg development and hatching in our rearing rooms pushes us to find solutions to regulate it, especially during dry periods.

I use an ultrasonic humidifier, adjustable and ideal:

humidifier Tao Tronics

I also use, either in parallel with the humidifier or when there is only a need for a small adjustment of the humidity level, a much less expensive solution: Ceramic saturators (around 5 euros per unit in DIY stores) :

I hang them on the oil bath radiator (which dries the air less than electric radiators of the radiant type or commonly called "Toaster").
The principle of evaporation caused by heat from the radiator works well, however it takes about a day to see the effects.

Saturator on radiator

I monitor the humidity daily and have set myself a minimum of 55% humidity in the air.

Hydrometer and temperature indicator

Below this humidity level, I therefore implement one of the solutions presented above or combine the two.
If I notice that the humidity level is not rising enough, I turn off the air extractor or regulate it. The air extractor being connected to an extractor speed variator socket.

dimmer

I avoid the solution of the bath, which could for me "heat" them and see them destroy their spawning to undertake another. Except possibly the day before the presumed hatching date, either by offering the couple a bath or by spraying the eggs.

The combination of heat + humidity gives a more favorable ground for the development of bacteria. And therefore diseases. Above 70-75%: I regulate the humidity and ventilate the room, if necessary by means of the air extractor to lower the humidity level.