I'm not sure what the "mathematically correct" way is to play these, but I treat them like any other tournament with regards to short stack play. In those freerolls everyone gets short stacked very quickly in terms of the number of big blinds each player has. Therefore I use my standard short stack criteria and I've placed in quite a few of those. I have 240 chips all from freerolls, I haven't taken the time to play anything with a buyin yet there.