How to get the Post position in a thread (post #) for a given postid?

[Lynne]

Users can set their own number of posts per page. Also, some posts may be moderated, and some may be hidden (user in coventry or a user in your ignore list) and some could be deleted. Different people may see the post as a different post number. Additionally, different users may have their posts per page set to different numbers.

What exactly are you wanting to do and perhaps we can suggest a different way of going about it (because your way is not easy).