I was nosing around the Internet looking for a Shotput Ventures term sheet tonight.  Doing a little diligence if you will.  While not officially associated with Shotput in any way I went and presented over the summer, went to demo day, and am talking to a few of their companies about various things that might help move them along.  My googling led me to Hacker News and this snippet of discussion:

This assumption is off by an order of magnitude. YC funded 61 startups in
Boston. Only 3 are still there, and 1 of those is talking about moving
to SV. So the assumption should be more like 2-3%.

For those that do not read HN, pg is Paul Graham.  He is a founder of Y Combinator, on which Shotput based its model.  The comment in italics is what an Atlanta hacker said about the TechStars companies staying in Boulder.

Now that the first Shotput session is complete we can put some real numbers on the matter.  There were eight startups in the Shotput class.  Of those three are still in Atlanta.  It’s early, but that is 37%.  The 20% assumption was indeed off.  It was low.