Looks like about 19 days in a row that the A share price has been below the likely price of the buybacks on March 4, for at least part of the trading day. So I imagine they have been in the market buying, at least some.

I assumed that the buyback that day was around the average price of the day, which was about $729500.

For this purpose, a conversion of 1500:1 for B share buybacks doesn't seem to be appropriate. Management prefers A shares for buybacks and doesn't seem to buy B shares unless prices are both cheaper (relatively) and quite cheap (in an absolute sense, perhaps to take advantage of the liquidity to buy more shares). So I would NOT assume that management would be happy to buy B shares at 729500/1500. My guess is that a notional "happy with a B buyback at that time" might be more like (say) $475 per B instead of $486.

A share is worth more each day, so one might assume the target rises commensurably over time. But it probably also moves up and down even more sometimes with changing prospects of other opportunities, present and future.

Jim