Morwenna
So, because magic circles are personal creations of temples I've been looking into actual physical temples and I found a few interesting things which go way back past the books I've been reading.
On the subject of where to start the circle- many witches enter circles from the NE; the avenue entrance to Stonehenge at the Heel Stone is in the NE- so aligning with the mid-summer solstice. Maybe this set the precedent?
On the 3 concentric circles within squares as seen in Key of Solomon, the Borobudur is one of the earlier examples of this - https://upload.wikimedia.org/wikipedia/commons/2/27/Borobudur_Mandala.svg
I never thought about Stonehenge in this regard but you may be right.
That Borubudur: I notice that one corner is different; it sort of looks like there are stairs. I wonder what direction that is in? What's the orientation?
Now you've got me thinking... (always dangerous!)