Your reckoning is correct. The $Billion comes from imputing the current value of WAMUQ value(.09)with 1.7 Billion shares (=153M) and than quantifying the Ltws as equal owners of the 30% allocated for this class ($153m/113m.). Together they would be worth ~$300M and therefor the whole company would have t/b valued @$1B.
In any case the book value as you calculate is not the same as the market valuation is today. All of my valuations are speculator induced. And, at any rate, this saga is not over yet.