A chap called Jack Morris Eyton has been researching the shape & positions of stones within large circles for a while. Over on TMP he produced an 18 part series of articles based on his findings. If you're interested, you'll find them on the following link under the heading “Last 20 News/Article Submissions from JACKME”
http://www.megalithic.co.uk/user.php?op=userinfo&uname=JACKME