Oh, I got nerdy too a while ago.
The 3113 BC date for the fall of Atlantis comes from a single source, the "Humans and the Cycle of Magic " address by Ehran (which exist in at least three different versions, 1989,
1990 and est.
2004).
Ehran says the following things:
- The average time between Threshold Levels is approximatively 5200 years
- August 21, 3113 BC marked the end of the Fourth World and the beginning of the Fifth
- Converting the Mayan dates to the current Christian calendar, [the Mayan calendar] correctly states that the Threshold would be passed on December 24, 2011.
- The Sixth World will end, according to the Mayan calendar, on April 4, 7137 AD.
The thing is, the Mayan calendar actually says the world would change after 13 baktuns, or 1 872 000 days, but there were only 1 871 270 days between -3112-08-21 (*) and 2011-12-24, and there will be 1 871 969 days between 2011-12-24 and 7137-04-04.
(* using astronomical year numbering, 1 BC is year 0, and 3113 BC is -3112)
So any calculation may be off by a year or two, if not more.
There is no known way to adjust the Mayan calendar to find these dates. Ehran's exposé actually got almost every fact wrong (at least IRL). If I was still freelancing, I would probably retcon it into a test he made to detect the most brilliant of the YET members who would have caught them on the fly.
Also, Regular Length and Middle Peak are assumptions that are not backed by any source. On the other hand, they make sense as working hypothesis. Your estimate is as close as one can get I think, but really, any SR author could give a wholly different date for the Earthdawn flight, and it wouldn't legally be a retcon.
But the more recent
Dawn of the Artifact: Midnight does mention 9564 BC for the destruction of Atlantis, this time completely contradicting the aforementioned source.