I think the point here is that there is no real accurate way to measure humidity at any single point of time. I think you can have your incubator to dry but not too wet, (to a point). The higher the humidity at hatching the better as this softens the egg shells and makes it easier for the chicks to get out. It is better to add too much water than not enough.I'm guessing your hygrometer is pretty accurate. The time frames are to let the isolated environment reach equilibrium. If the airspace is small enough, and the amount of damp salt large enough in relation to the airspace, it could probably reach equilibrium sooner. The 8 hours is more to be sure that it has done that, and I suggest waiting the recommended time to see what it shows at that point. But you might have scored a pretty decent hygrometer.
Good hatching everyone.