While this is an upper bound for a "board position", it should be noted that it is not an upper bound for a "game state". That includes the (unbounded) whole board position history because of the threefold repetition rule. If you ignore that (and the fifty-move rule which can alternatively be kept using a six-bit counter), you also need the castling state and the en passant state. Plus one bit of the player on move, obviously :-)