The Z-Stones were reconstructed by Keillor, if I remember correctly, and that's where the error will lie, I should think. There's enough alignments to the southernmost moonrise at other sites to, at least, expect one at Avebury. Without the southern circle the Z-stones are the best possibility. Still not found the barrow(s) on Waden Hill that are in that direction, though, nor identified the species of tree in the copse. Ultimately it's not important!