Not sure of this....(edited after I thought about it for a while longer)
Using a Z-score table .15% from 100% = 99.85%
99.85% represents 2.96 S.D. from the mean
so +- 2.96 S.D. represents the 'middle 99.7% of all candy bar weights'
2.96 S.D. x 2 gm/S.D. = 5.92 gm
SO acceptable weights would be 58 +- 5.92 gm = 52.08--------63.92 gm acceptable weights