I've had similar problems and luckily have not had to shoot the dog....yet. I had a pit charge me one time and I knocked the crap out of it with a bag I just happened to be carrying at the time. He wobbled off. Get you a WASP or pepper spray. I usually carry one or the other with me now when I walk my German Shepard just in case another dogs off a leash charges at us. I would not hesitate to shoot it if I felt really threatened. Thankfully, I grew up with dogs so I'm not overly scared of them. I did have GA anilmal control out years ago and specifically asked him this same question....at what point can I shoot. His answer was whenever you feel threatened, but to make sure I killed it on my property. If you shoot it and it goes home and dies you'll have a lot of explaining to do. Good luck brother.