I'm copying the IFEFFIT mail list, they may have the information buried in the archives somewhere.

That link points to Grant Bunker's webpage containing his XAFS tutorials and it will not open for me at the moment either. However, there are several very good books that have come out lately on XAFS that explain self absorption quite well.  If you're not interested in getting a copy of the books, there is a discussion thread from when I was a newbie to overabsorption that is found on the XAFS.org tutorials page.  I suggest going through it as several contributed substantial knowledge in that thread (http://www.xafs.org/Experiment/OverAbsorption). 

Cheers, 

-Rich