So, this is my 5th hatch of ~100 this year. Things are getting better, I have an 80% fertility rate and an 80% hatch rate, so 64 out of 100 now. I'm hatching my own eggs, creating generation 2 of my own breed. My layers have come back into their own and are now giving me 25 or so (from 38 layers) a day...so I'm able to incubate nothing but fresh eggs (e.g. < 7 day old eggs). I have been incubating eggs < 63g in order to fit 98 into my Ova 100 (49 per tray). I'm not suggesting that larger eggs wouldn't hatch well, only that if I use larger eggs I have to set fewer eggs.
I have stopped weighing my eggs during the incubation...in fact, I have tried to stay out of the bator as much as possible. I run it at 51% RH and 99.6F. I candle on day 7 and day 10 to remove obvious clears. At the beginning of day 19, I put the eggs onto trays that have a paper towel liner, check for Red Rings (early death eggs) and bump RH up to 65%. After that I only open the bator once a day. I run my hatches for 23 days, tossing anything unhatched on the morning before day 24 (which starts at noon for me), allowing me time to clean the bator out before Day 24 begins.
I am not hatching for pets, but for production birds (eggs or meat), so if they have not hatched by themselves by Day 23 they aren't what I want to propagate. Sally Sunshine has a great treatise on assisting if you are interested. http://www.backyardchickens.com/a/step-by-step-guide-to-assisted-hatching
Have fun, but keep your posts about your hatch please.