They can buy back the 25% of One Beacon they do not own, pay a one time $85 dividend, buy back 2 million shares at $395. That would take book value to $530. There is further upside to book value, where it should be selling. After all, BV is cash and securities and the book of business is worth a lot. Insurance companies usually sell at 1.25-1.5 times book and will get there again.
Tangible book value is a fluid sort of concept - like everything else in the WTM accounting universe, the effect of this transaction on tangible book value will be whatever your management says it will be, and you will like it.