Well, not knowing how far along puts a big hinderance to sound advice, so the advice that you are given you'll have to make your own judgement call on wether to use it or not based on what you are experiencing with the chick. In an unharmed egg (in cases where you hear peeping within the shell) it is after the chick makes it's internal pip. Usually somewhere between a few hours to a day before you see the external pip. This is what I would do. Not knowing what day they were on or how advanced into the hatch process it is, I woud take a wet paper towel and wrap around 3/4 of the egg, leaving the spot where the shell is missing upright and replace him to the "incubator" and give him time to see what happens naturally.I have a mess. I have multiple eggs that were added to a setting hen's clutch, so no way to know what day/stage they are at. The hens traded spots a time or 2, so there are 3 chicks hatched already and that hen has stopped brooding the remainder. I brought them in to my really sorry attempt at an incubator to try to finish the remaining 3 eggs. Worst planned chickraising scheme ever!
okay, now that that is out of the way, one of the 3 remaining eggs looked like it got stepped on or otherwise smacked on one side, when I found it there was a miniscule spot of dried blood on the membrane and lots of shell fragments gone from around the edge. Chick is alive and peeping. I'm trying to keep humidity high, misting. covering, keeping warm. I do have a combo temp/humidity gauge in there, but it has been drier than 30% occasionally, and I try to bring it up to 65 or more. Really bad setup, I know. What can I do to help this chick, how would I know to stop assisting if he's not ready, and how long can they be peeping ahead of final hatching readiness? Thanks for swallowing your disgust at my ineptitude! Feel free to chew me out along with advice, I promise not to be offended!
Now, if you assist. You know it's time to stop when there is active bleeding from the veining in the membranes and/or you spot unabsorbed yolk. Assisting should be done slowly and a little at a time replacing the chick to the bator (I prefer to keep a wet/damp paper towel around the back of the egg.) for periods of rest and to give the chick a chance to make it's own progress.
I wish you well, but realistically, w/out knowing the incubation period he's at, without knowing how much damage the chick itself has sustained and w/o a proper incubator with constant adequate humidity, the probability for a healthy hatch is pretty low. But that doesn't mean we can't try.