I haven't changed the software in years. The index numbers are relative to the present and don't correspond to an absolute range of threads, so basically you just count up from 1 (the current TOC).
Just to be sure we're talking about the same thing, you're manipulating the URLs like this, right?:
http://www.habitablezone.com/nexus/toc.asp?TOCpage=3&svcpt=community
That's pretty slick hacking, because the site doesn't use the URLs that way; the parameters are usually sent in a form, but appending them to a URL is an alternate way to do the same thing.
Just in case I misunderstood and you're talking about message ID numbers, those should generally work. If a message isn't already on the server in the form of an HTML page, it will pull the message text from a database and create one on-demand. The only way for that to fail (when it's working right) is for the message to have been removed entirely from the system, database and HTML versions both. The pattern is:
http://www.habitablezone.com/forum_name/messages/message_ID.html
e.g.
http://www.habitablezone.com/geek/messages/500459.html