Going for Two. Washington at Atlanta December 15, 2013

Washington coach Mike Shanahan gets the opportunity go for the win in regulation and takes it, going for a 2 point conversion instead of electing to take a PAT and take his chances in Overtime.  QB Kirk Cousins misses the pass and Washington loses to the Atlanta Falcons 26-27.  Regardless of the result, was this the right decision?

The Situation

Down 20-27, Washington QB Kirk Cousins completes a TD pass to WR Santana Moss, pulling the team within 1 point.  There are 18 seconds left in the game, so whatever happens here is highly likely to be the final outcome of regulation.  Coach Shanahan decides to go for a 2 point conversion.  Was this the right move?

This is a relatively simple situation to analyze since there are only a couple of possibilities.

If Washington goes for the PAT, they will go to overtime 98% of the time historically.  That's close enough to 100% for our purposes.  In overtime, there is nominally a 50/50 chance of winning the game.

If Washington goes for the 2PAT, they will convert and win the game x% of the time and lose the game 100-x% of the time.  What is x actually?