When the chick starts zipping they make rhythmic cheeping sounds and thrust the beak through the shell and then rotate to poke the next spot. You may have watched this on the others that hatched. If this one gets to that stage but continues to poke the same spot on the shell, without rotating, then you know its ready and needs help. I like to make sure it is truly ready by watching it repeatedly poke the same spot for a little while, but not too long. Watch for yawing and chewing motions that indicate the yolk is still being absorbed.
In my experience, a pip hole that big that has been that way for hours and being the last one to hatch, this chick might need you to help. I would check on this one every 20 mins until it was out. A pip hole that big for too long sets up the risk of the membrane drying out. You must wait for the zipping transition. The chick will show you its ready and struggling.