Here is the easy version of what I posted.So you are both saying there is no way to go to post #643. Wouldn't it have been easier to say you can't do that with XenForo. I already knew how to get to page #382 but that wasn't my question.
You want to go to post #643 so you divide by 20 and get 32 plus a remainder, which means it's on page 33.
Go to the address bar and change the page number to 33.
This is not "there is no way", it's just one step more than entering the post number itself.