No it is most certainly NOT reasonable to think you can make a 6:15 flight if you leave after the game. This was brought up sometime last season for someone and you may not even be near the airport by 6:15 depending on conditions leaving the game which are sure to be slow and then if you have a long security line... I think someone said they made it for a flight in your time frame so while perhaps it's doable, it is very much on the un reasonable side of expectations.