I use egg weight loss, in my waterbed incubator I do dry hatch, but even after a few hatches I still monitor egg weight loss to be sure I'm on track. It is very easy to do and I don't care if you dry hatch or not if you monitor egg weight loss you can adjust BEFORE it is too late. You can draw lines around the air cell but by the time you can be sure that the weight loss was too much or too little it is too late to do anything. Here are my simple instructions I wrote for incubators I built. If you are really committed to dry hatch but your eggs are losing weight to quickly, aside from adding water you can vent less or turn the temperature down slightly, if you are hatching with added humidity, just adjust the humidity.
I'm having trouble finding any simple written instructions for monitoring incubation humidity by egg weight loss, and yet it is so simple and I would never want to rely on a hygrometer again and when a hatch goes badly, I want to be confident that it was or was not a humidity issue.
Day#1...weigh every egg individually on setting, and write the weight on the shell (eg 56g)
Periodically during incubation weigh 5 or 6 random eggs and calculate the weight loss.
Eggs need to lose 12-13% of their weight by day 18 of incubation, but just to keep it easy, use 12% which works out to .67%/day and understand that anything in the range of 0.6-0.7% is good. If your weight loss is higher, then you will need to increase your humidity to stay in the optimum range, if the weight loss is lower, you need to decrease humidity
Anyways, many instructions I see say to batch weigh your eggs on day one, I say to weigh them individually on day one because at some point most of us like to remove quitters and non starters from the incubator so if you have a batch weight for 40 eggs and on day 10 you take ten eggs out, your initial weight isn't going to work anymore!