Give me more info. That's from South Station to Airport or on all five lines of the Silver Line?
From the Bluebook 2010, it looks as though you're including SL1 and SL2 and the "Shuttle" in that total. I've never been able to find out what that shuttle is. Where does it go? Why is it offered? Why is it so much faster than the SL1 and SL2?
Why does the MBTA run a street-level shuttle when it has a Silver Line bus line on the same route?
http://www.mbta.com/uploadedfiles/documents/Bluebook 2010.pdf
I'd correct your number by saying 15,000 people aren't riding the Silver Line to the Seaport District. 7,500 people are going round trip to and from South Station.
Also, I'd subtract the "offs" who come in from the airport to the Seaport District - those aren't employees but people coming in from out of town. Likewise, I'd subtract the "ons" who get on the bus in the Seaport District and head to the airport. Those are business people on their way back out of town.
Where is the closest bus stop to this new PwC building at Seaport Square?