Assuming people do not strategise around the two third concept too much, that number would obviously be 66.66~. However, since I expect most people will strategise taking this into account, logically, the best chance on guessing this right would be two/third of two/third, which would be 44,44~. I am also guessing that the amoutn of people who realize this is negatable against the number of people who don't take the two/third notion into account at all. My guess will be:
Although my explanation obviously will render my point mute.