This is an excellent point, actually. I know for a fact the hygrometer that comes with my Incuview isn't perfect - when I tested it it came out about 10% too low, and it loses specificity below about 18%. However, it's consistent with itself (determined by comparing it to another hygrometer), so I calibrate the humidity in my hatch based on weight loss. If I hold it at "30%" until D7, but they haven't lost the correct amount of weight by that time, then I'll drop it to 20% or 25% per my hygrometer, for example, and vice versa. Is that a true 20 or 25%? Probably not, but it doesn't matter.
Another thing to consider is that humidity is really about the average over the length of incubation (prior to hatch). It doesn't really matter if it spikes to 50% for a couple of hours one day, if it also drops to 10% for a couple of hours. And unless you've got a fancy humidity pump, or are using a hygrometer that has recall, it's going to be a lot harder to figure out what your average is.
Sorry, didn't mean to take the wind out of anyone's sails. My science-brain just likes to be difficult.
