That's post number -- there's no mathematical formula for mapping post number to thread number without knowing everything there is to know about the site. So the only way is to try it.
Try something like this:
site:head-fi.org "Dusty Chalk" feedback
and then instead of clicking on the links, click on "Cached".
I only have one page of feedback, and it's the first one in my search; you guys may have more work ahead of youse.
Justin, I can't figure out the exact search, but if you can figure out some text that shows up in peoples' public profiles, you can replace the above search's "feedback" with that text. "home page" "private message" "location" "occupation"? I'm not coming up with it.
Oh, never mind, I found one, and all the usual public profile stuff like MSN, ICQ and whatnot isn't in there anyway.