Either sum the ratings of all participants and divide by the number of participants. I don't know beyond that, but the figure is fair, though I don't know what to do with it—I'm not great at math :(
Added after 2 minutes
I'll also mention the pros of the system. After all, tournaments can be based on a main objective for resource gathering or experience, where the duration is equal. Well then, just plug the amount of experience or resources into my formula instead of days and calculate the rating further.
