Be careful with candling-- Every time you open the incubator to do it, the eggs' odds of hatching are decreased as cold air rushes in, moisture rushes out, and the eggs are likely to be jostled a fair amount or rotated past the 180 degree radius that is best.
Candling once at day 8 has worked well for me. By then, chicks are far enough developed that you can really tell. And few enough days are left after that I think if a chick dies after that, its egg won't have time to get so rotten that it will explode (which can be a gooey possible danger to good eggs if a dead egg is incubated too long).
For candling, I recommend a pen light or other small flashlight held right against the egg on the opposite side from you. Most of these flashlights don't get very hot so don't pose as much of a risk of overheating and killing chicks. A small flashlight has been the ONLY light that's enabled me to see well enough through blue and olive green eggshells.
Best!
