I use my historical journals more than the zone approximations. Thus, in central Ga and just south of Atlanta (where I've lived) I use April 15th as last frost date. Last year our last frost was on April 14th.

Consider, too, what you are trying to protect. Brocilli, potato, radish, sweet pea etc. will all take a light frost just fine. Peppers, tomato, squash, melons can take none, not even a light frost.