I can’t find the link but somewhere in the Minyanville articles on this crisis they previously pointed out that there is $700-800 billion in corporate paper to be refinanced this year. That may be where Paulson got the $700 number from.
http://www.kitco.com/commentary/old/roulston/roulston_sep272007.htm
this article has a brief description of how mortgages of various default risks are stratified within a mbs.
Thanks for the link. From the article, it really does seem like Paulson should be able to put out numbers that define the scope of the problem. Mortgages totalling X are in default. Mortgages totalling Y may go into default. Mortgage totalling Z will probably not go into default.