That is simple as most only track weight until the last candling at lockdown. Few weigh the chick and shell at time of hatch after they've spent another 3 days or so in the incubator, yet if you do candle, the drawdown of the air cell is evident. 15-16% are the most commonly published figures for at time of hatch. If yours are 20% light, then there is obviously an issue.
The mother hen normally acts as tour guide for the young. As we are interrupting the process, it's really not asking too much to point them in the right direction. Compared to robins, dogs, cats, rabbits and humans, chicks aren't too bad off. They are quite low maintenance. At least we don't need to teach them to fly.
Ok, gotta ask for a clarification. Are you saying you think its ok to take shell and presumed hatchling from that shell, and weight them while still wet (and potentially there are zips and pips going on)?? I could get the shell and hatchling out in like 5 seconds, and my styro bator wouldn't drop 1% humidity, so I can see how it shouldn't adversely affect anything. But presumably you would only do that with chicks that have some obvious issue?? And so you find under or over-weight, what do you do then? Put them back in the same bator...in like an hour it will be impossible to figure out which chick came from what shell...