But that's using closing figures, which aren't the best way to measure it.

I don't think you can use ANY figures to properly measure it. That's because the As don't trade nearly as much as the Bs, and the As have a rather wide bid/ask during the time that they aren't trading. For example, right now, the bid/ask is $655,329.91/$655,995.00, and BRKB last trade is $437.48. So the ratio on the bid side is 1498 and the ratio on the ask side is 1499. But the bid and ask constantly are changing even though none of the As may be actually trading.

Now, a few minutes later, 3 As have traded (at $656,785.01) and the bid/ask is $656,460.03/$657,119.88, and the Bs are constantly trading in quantity, now at $437.57. So 1501 ratio using last trade, 1500 at bid and 1502 at the ask.

The other thing that may influence the number a little is the fact that the As have no options and the Bs do. Sometimes option trades can influence the stock price as large option trades are settled, and the As because they trade much less frequently only catch up to the Bs (ratio-wise) a bit later.