Yes, the hills are the starting off places. The ideal was almost hemispherical, certainly in the Pennines, so it is easy to impute a fecund association, and a need to promote that association.Wheat certainly didn't produce at four tonnes a hectare as it will here now. As there were no instruments in prehistoric times capable of measuring closer than about a degree of horizon it is pointless to look for modern accuracy in monuments. And, yes, it is impossible to determine where the centre of an irregular circle actually lies - this will have been lost with the rest of the oral lore. Remembering Swallows and Amazons (based in Cumbria) and their method of navigation, it is surprising that any has survived at all. More has been passed down through the local masons, of course.