I'm a bit curious as to how you came up with 1/28 of a soul. Horcrux one would be half a soul, half of that is 1/4 for Horcrux two, half of that is 1/8, 1/16, 1/32, and finally 1/64 for the sixth horcrux, leaving him 1/64, if you're halving halves as Harry said.