The reason you weigh your egg is to determine weight loss during incubation. This means you would have to weigh an egg before you put it into the incubator and then weigh it during the incubation process to check if your humidity is right.
Eggs should actually LOSE weight between the time you start incubation and when they hatch. Ideally they should lose about 13% to 15% between the time they are first layed to the time they pip. So, if you weigh your egg before you put it into the incubator and it weighs 2 oz. it should weigh about 1.7 to 1.74 oz at the time it pips. This means it should weigh between 1.87 and 1.85 at day 10. If it weighs significantly more or less, you need to adjust your humidity. (I would venture that "significant" is measured in the .01's)
The humidity of the incubator environment affects how much moisture is lost by the incubating eggs during incubation. The more humid it is, the less moisture - and therefore weight - loss occurs. This way you know if you need to add humidity (egg is losing too much weight) or decrease humidity (egg is not losing enough weight).
The reason you are concerned about egg weight and humidity is that the egg needs to develop an air cell inside that the chick breaks into or "pips" during hatching. If this area has a lot of liquid in it (the egg did not lose enough weight), the chick will not be able to breath and will drown. If it is too dry inside the shell, the chick has difficulty spinning around cracking open the egg to hatch out.
Weighing an egg for the first time after days of incubation only tells you how much the egg weighs after days of incubation.
*To those who have far more experience hatching: If I am inccorect. please correct me as I always welcome the opportunity to increase my knowledge*
