Eggs need to loose on average 12-14% of moisture during incubation, to create enough space in the air cell and prevent the embryo from dehydration.
The moisture loss of an egg is the result of the conductance of the shell (the "resistance" of the shell against gas exchange), and the water vapour pressure deficit, the difference between the water vapour pressure inside the egg and outside the egg. This water vapour pressure is the result of the relative humidity and the temperature of egg and air. The relative humidity inside an egg can be estimated at 99%.
The conductance of an egg doesnt change during incubation. That means that if egg and air remain constant in relative humidity and temperature, the moisture loss will be constant as well.
Although practically the temperature of the egg rises during incubation as a result of metabolic heat production, this will not have a big influence on the absolute moisture loss.
Also the temperature of the air doesn't change dramatically during incubation, especially not in a multi-stage machine.
This means that if relative humidity is kept constant, moisture loss will be constant as well, and the total moisture loss can be calculated if for instance the moisture loss is determined after 7 days of incubation.
However, moisture loss doesnt need to be kept constant for a correct development of the embryo. As long as the optimum total moisture loss is achieved after 20 days of incubation, a temporary increase or decrease in moisture loss doesn't have an influence on the hatch result.
This allows us to change ventilation settings during incubation, and cater the circumstances for the embryo better during the total incubation process. This is ofcourse mainly possible in single stage incubation. Multi stage incubation requires a much more continous and stable climate throughout the incubation process.