I ONLY candle my eggs at day 18 when the turner comes out and the eggs go into the carton for hatching.
I have found that less handling of the eggs gives one a much better hatch rate.
I candle as quickly as I can and back into the bator they go.
I leave the chicks in for no more than 36 hours and no less than 24.
The egg carton method allows one to keep the bator closed without the problem of the newly hatched breaking or moving the other eggs.
They zip a lot faster and it really is much less time consuming and less of a hassle to do it this way.
Trouble is you lose a lot of fun an entertainment, but I have done this enough times now that all I really want is a nice hatch out.
In the old days they used to make bators with on board candling.
Now it is just too expensive.