Why would the life be lost when emerging? The air sac is either large enough for a chicks beak to breath in after internal pipping or it's not.
I've drowned chicks, it's actually quite hard to do. It takes reading and believing a ton of misinformation on the internet that states 60% humidity incubation and 80% humidity for hatching. This of course is a gross amount of moisture which even if there is a tiny air sac it's likely filled with water. My first incubation attempt years ago this happened, 15 of 18 were healthy and about to hatch day 18 and only one made it.
There is a surprising range of conditions chicks hatch from. I've read of people who swear by 50% RH in incubation. That's not my findings but who knows? Many people don't calibrate hygrometers so you can't believe much in way of exacting numbers for RH. I do calibrate my hygrometer prior to each hatching season. The best hatch rates and no adjustment of humidity for good air sac growth I get with under 35% RH, it's hard to get much lower than 30% RH with an incubator full of eggs emitting water so 30-35% is the range for me and that's given 100% hatches. For hatching I up humidity to 70% late day 18 or early morning day 19.
If your worried about things you can always run dry for two days. Take all water out then put back enough to get 70% RH late day 18. I doubt you'll have problems the way you are now though. It sounds like your air sacs are of good size.
Sure you've seen these diagrams but here's another. The diagrams are meant as an aid not exacting measurement. Weight loss and size of air sac are the same thing. One is more exacting than other yet both are ball park measure with averages used. One can't get every egg to loose exactly 12% mass. Heck, here are two diagrams and one can see they are not equal.
Edited by Egghead_Jr - 1/25/16 at 6:12am