I only tried it briefly last night so I'm not sure about this, but probably (circumference of the earth) - (straight line* distance between guess and actual loc

I only tried it briefly last night so I'm not sure about this, but probably (circumference of the earth) - (straight line* distance between guess and actual location) A bit more experimentation should reveal the real answer. *I'd have thought they'd use great circle, but no: it's just a big straight line on the map.