When I have a hard time trying to determine the state of a dark egg I put the candler under the egg so it lights up the bottom. It seem that the good eggs have some lighter area under the dark blob. If most of the egg is clear it probably stopped developing, and if it's all dark it's probably going rotten ( sometimes ready to explode.) If all of your eggs are set at the same time it's easier to determine what the good eggs should look like by comparison.
So when do you make the decision to pull eggs from a nest? Is it the same as others? "Stinkers" or "oozers" (and obvious blood rings) get removed. At what point do you say, "I see no veining in this egg, gotta take it out?"