Is there a placeholder for the true value of WM/SharedUserRating?
It is not %rating mm% or %rating wmp%. These are for application-specific star value interpretations of WM/SharedUserRating. (Note: %rating mm% is currently bugged)
It is not %rating%. This maps to a different WM/ field (presumably the obsolete/obsolescent WM/Rating). And I find that RATING is absent from the Tag Fields Mappings http://help.mp3tag.de/main_tags.html and from placeholder pickers e.g. http://i.imgur.com/BXqAuL6.png .