Some people are hesitant to "go to the gun" and either dither or just freeze up at the last second. Others are too quick and fail to use all their options before firing. There's no cut and dried answer as each confrontation has it's own unique personality.
Shooting an uncontrolled, aggressive dog would seem to be fairly easily justified legally. The only second guessing I'd do is suggest you might have made a tactical error by not reentering your vehicle and calling animal control(if there was such an option) instead of heading for the porch although "gaining the high ground" is logical.
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