It's not quite as easy as that. I haven't heard of a % rule before, although that's not to say there isn't one.
Basically, a product needs to be fat FREE (not low fat) to be a free food but, as I said in a previous thread, just because a yoghurt is fat free it is not necessarily a free food (yeo valley vanilla fat free has syns). Some fat free foods have added sugar (not sweetener).
Fat free natural unsweetened yoghurt is free. Fat free cottage cheese is free but low fat is not. Some flavoured varieties are synned.
It's not straight forward!