Well the final version of Stonehenge was circular - no need for measurement, You decide what radius you want (by thumb/eye) and build around on it. And if it was a calendar of sorts, which is likely for an agricultural culture, a calendar based on the horizon needs neither numbers nor distances - it relies only on relative positions - it's analogue.

I visited the Rollrights about 2 years and 2 weeks ago today - It was a lovely memorable day - and the fields were golden, ready for the harvest. We counted the stones round and round several times and never got the same number lol, just as the tradition says!

Dave Light