I started a test here - Keyword Position v CTR
Please email me your Google WMT data for CTR v Pos - I don't need names, URLs or even keywords, just a two column list with CTR and Avg Position from your Google WMT data. You can email it to me at firstname.lastname@example.org, paste it in this thread, reply to any related thread, PM me etc.
I will publish the results of the test which should show what the typical CTR is for any given SERP position, and (more to the point of the test) show the likely ranges that you should expect.
Thanks in advance!