No. of Recommendations: 1
Unless my table missed something, 1.4796 times "peak to date" known book at start of the month of the buyback seems to be the absolute top ratio paid for a B share (again, average within the month--presumable a few shares a bit higher and lower).
Another quarter in support of your point -- the stock dipped down to the low $440s a couple times in Q4 and Jan 1 - Feb 10, but we have zero buybacks in that span.
If past A purchases were the best guide, we probably would have seen at least some repurchase activity -- the buy trigger would have been about $457 based on Q3 book. If the top ratio paid for B is the better guide, the trigger point was ~$432, which was never hit.
New B-guided trigger point would be around $445.