Note the current count, post a post, hover over one of the links below your post that show the ID, such as "Delete" or "Reply," calculate the offset from the current count to 6,000,000, and add that offset to the ID. That will be post 6,000,000. The path to the post will be
http://trekbbs.com/posts/<ID>.
So posting this to get the ID and while the current post is 5,999,307, I get:
http://www.trekbbs.com/posts/11529794 will be post 6,000,000. This link will get an error until the post exists.
I did this late at night while traffic is low and confirmed the post count had not changed while doing the calculation.