Hi! There's a fair bit of "inventiveness" associated with this topic. Possibilities are explored is great detail at http://www.geoffss.plus.com/perpetualchoirs.htm
There is an almost forensic dissection there of how the 3 original Triad sites of Old Sarum/Salisbury, Glastonbury and Bangor-on-Dee got morphed, by 1823, into the 3-some approximating to John Michell's Stonehenge, Glastonbury and Llantwit and, thus, into his decagon idea.
What is surprising, as reviewed at http://www.geoffss.plus.com/movingmenhirs.htm
is how John Michell seems to have wandered into someone else's New Jerusalem creation and - at least in his earlier version - created a model (had he known it!) that finds Pi and Phi (and St John) as Latitudes.
Best
geoffss
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