"He used to say shares should trade below 1.1x book, then he said it was 1.2x book, and most recently he has just said they need to be below intrinsic value, conservatively calculated."

I don't think the 1.1 and 1.2 were really about the value of the shares.

I think those were set back when Buffett still had his hang-up about share buybacks as him taking advantage of the sellers. The 1.1 and 1.2 price/book thresholds were just a point where the value was so ridiculous that he couldn't pass it up. His announcing the thresholds beforehand helped him assuage the guilt he felt as an insider taking advantage of sellers. It is quite obvious that his thinking about buybacks has evolved over the years, not just in terms of value, but also in terms of ethics. I think those buyback thresholds were just evolutionary stages in his thinking about the ethics of buybacks more than they were stages in his valuation of Berkshire.