I've gotten way too into candling eggs. After last year ("They won't hatch...all I see is a big dark thing and then a clear spot." "Um, that's the chick. And the air sac.") I thought I'd given up on it, but this year I have developed some skill (I think) and between looking at ag sci university sites I candle the eggs.
According to Poultry Scientist Me, of 8 eggs 6 seem to show development, one isn't clear and the final egg has a blood ring. I think.