On further reading of his blog he eventually got the f ratio down to f5.1 with excellent results. See
http://www.davidcortner.com/slowblog/20120124.php
That represents a distance (total) of 149.5mm to get to that f ratio. His stars were very good.
He finally stopped at a f ratio of f4.8. See
http://www.davidcortner.com/slowblog/20120225.php
Hope this helps
Allan