Good point. I'm looking into it right now. At the moment, I think the solution is to consider the estimated odds being displayed at the time of betting. Since, that is the odds that made the bettor place a bet on that event. I will talk to everyone who's betting on freebitco.in and see what they think of it.
Estimated odds are based on the current amounts placed on the outcomes by the bettors and not on the likeliness of the outcomes to occur as for classical bookies. That means, you can get estimated odds at 2.5 while on all other sportsbooks the odds for the same outcomes will be 1.5 for example. So it would be very easy for freebitcoin players to chose the overvalued "estimated odds".