I expect he buys back when they have excess cash, and the price is within a conservatively calculated range. That range might be +/- 20%.

I think this is correct. He has repeatedly said that he does not use or advocate a mechanistic approach to such valuation or decision around repurchase.

I use 10% from my impression of his value seeking traits based on observing him as a shareholder for more than 25 years. I would consider a safe bet that he would not buy anything in size at a price where he didn't think it was selling for at least a 10% discount to his conservative assessment of value. This includes his own shares.
Obviously Intrinsic value is a range and it fluctuates so at best it can be a crude heuristic. In my case it simply indicates that since Buffett is a buyer of his own shares at the current price, it is unlikely they are overvalued despite the run up in the last few months.