It was me who initially added the calculation of when the map was created. My source (which was uncited as we were not used to citations back then) was a post by Andreas Moehn ("Lalaith") in an old mailing list, known only as "The Tolkien List" (its address is/was firstname.lastname@example.org).
That was years ago and right now I can't find any homepage or archive of that group so that I can add a citation. The present state of the article pinpoints the creation to TA 2789, although I can't remember if that was the exact year Lalaith calculated. Right now it looks like "Original Research", but it isn't. If you even find anything concerning the whereabouts of the Tolkien Mailing List group please let me know. Sage 09:20, 13 May 2013 (UTC)