That goes back to the original problem - it's not supposed to be "gambling", it's supposed to be a matter of honor. You're not supposed to sit and calculate your odds of winning, you're supposed to roleplay a reaction of a medieval knight to an insult.
I think the issue here is almost entirely a role playing/player behavior one, and like it has been said, you can't really govern that. But you CAN start by setting the tone with your own characters.
As for the 5%/90%, those numbers are a terrible exaggeration in both directions.
I agree with Jens, the world is surely coming to an end.
