I thought themost important reason to stop rear facing was if the top of their head was less than 5cm from the top of the seat. Rating or no rating, if the back of the front seat can move back (in an accident) and contact with the top of their head, then you have a real problem.
So its kind of torso length+ the ability to push against the back of the seat with their legs and come within 5 cm of the top of the seat.
Thats my understanding anyhoo..