I know this is an old thread, but I think I might be able to help you. I know that this isn't the simplest way but would still work.
So, this is my 1st page on my build thread. Notice how the topic is 24465.0 because it is the first reply. http://transamcountry.com/community/index.php?topic=24465.0
Each page on the thread has 15 replies, So 24465.0 would be the first reply and if you wanted the second page you would add 15 replies or 24465.15 and so on. So if you wanted to get to the 15th page, you would have to do as follows. Since the first page started at 0, you would have to take 14 times 15 = 210. Or in other words whatever page you want subtract 1 and multiply by 15 replies and add to the end of the topic you are looking at. Once you have this number just replace the 24465.0 and with 24465.210 and that will take you to page 15. http://transamcountry.com/community/index.php?topic=24465.210
Not sure if that was clear or not, but should work for you. Hope this helps.