I have so little patience I should not be allowed in the hatcher room for three days around the due date.
If a chick has pipped, and has been pipped for at least 18 but better 24 hours without zipping, or has started distress peeping after 16 hours, or if the membrane starts turning brown and wrinkled around the hole, or has started zipping and stopped for more than 30 minutes, I go in.
And I am not dainty about it.
I will enlarge the hole a little with tweezers, needlenose pliers, or whatever comes to hand that can nibble at the edge of the hole. Holding the egg with the air cell up and the hole facing you, chip away the shell at the left edge of the hole and pull back on the thick white membrane. If you don't see blood or blood vessels in the inner membrane, keep going. Chip just the shell around the cap. If you haven't experienced any blood and have a struggling chick, you may want to let it try to push out as that will help ensure that the yolk is absorbed and that it has "pumped up" in preparation for hatch. Or you can just pull the top off the egg and help the chick fall out. If you do this, if the yolk is not absorbed fully, at a minimum you will end up with a herniated navel and exposed yolk. That's why the waiting period is so critical. But in the case of a stalled zip, the yolk should be absorbed and the chick just stuck for one reason or another.
I "save" most of the chicks I deliver, and I cull about half the chicks I save.