While it is true each state will have different laws, the universal truth is, if you feel like your life is threatened or you fear great bodily injury, you have every right to defend yourself. No if, ands, or buts about it. Because of this truth, it really depends on the person involved (just like all self defense shootings). I may handle a situation differently, being a 180 lb 5'10" muscular athletic male, than a 115lb 5' female.
German Shepherds are big, powerful, amazing dogs...but they can do a lot of damage if not trained correctly. In your specific scenario, I would not have shot, I would have fought the dog off with a less than lethal alternative. I have been around dogs the majority of my life, I study and rescue them as a hobby, and i have a lot more patients and training in dealing with dogs. I do not expect anyone else to share my feelings (although there has been a lot of members that have shared these same feelings at USACarry).
“One of the illusions of life is that the present hour is not the critical, decisive one.” – Ralph Waldo Emerson