In December Berkshire repurchased shares at a P/B of 1.48 (using Dec 31 BV). That's an all time high. Since Buffett has stated that he would not repurchase shares at a "mere" (his word) 5% discount to IV, it leads me to infer that his conservative IV estimate is at least 1.56x BV.

When the repurchase price was 1.2x book value, someone did a similar backward-looking analysis and asked Buffett why he didn't buy when the share price was below 1.2X book. He answered that he didn't think the price *did* dip below 1.2 times book, he said he'd been watching it very closely. It didn't dip below 1.2 the book value for the last published quarter, so Buffett was right, but the stock traded below 1.2x BV for the following report.
I'm sure Buffett has his finger on Berkshire's pulse and knows if anything catastrophic is happening, but I doubt he's asking all 56 subsidiaries to prepare the accounting and report the information required to figure out Book Value on a schedule more frequently than the quarterly numbers that need to be reported. It's simply too much work for too little gain. That means that the BV he had to go by for repurchases in December would have been BV reported at Q3 2022 - supplemented with information that nothing terrible happened since then.