I have come very close to shooting a dog twice, both times the charge came from within 30 feet or so, and both times the owners literally jumped on their dogs to stop them. Given a safe background, I would shoot as soon as the dog actually started to charge, or as soon as it was close enough to ensure a solid hit.

If worse came to worse, I have always planned on feeding the dog my left forearm, then gut shooting it a dozen times wth my right. I also read, years ago, that it is possible to feed the dog your left forearm, then rapidly put your right forearm behind its neck and push away with the left while pulling in with the right and break the dogs neck. I would really hate to have to try that one...