When this was brought up last time with Beleke, Bolt I passed on the question online via BBC to M Johnson which here anwered here if you access BBC link video, the transcript it there if not:
http://www.bbc.co.uk/blogs/tomfordyce/2009/04/michael_johnson_your_questions.html
He was fairly dismissive but in my opinion knowing sprint distances does not automatically make you an expert on longer races.
The originator of the KB/Bolt thread came up with a distance of approx 680m