I, too, remember when AA would always hold back exactly 2 seats, irrespective of the routing, the aircraft type, or any other factor. I do not recall, however, any discussion on this board of a 20% rule of thumb, and certainly have not seen any evidence of it in my travels. I agree with Fly AA J that the determination is far too complicated to try to capture in any such rule of thumb, and I don't believe that the OP's observations necessarily reflect any sort of change in policy or pratice.