Oh, I forgot to take into account that shorts can both partially cover and add more collateral.
So, it's not as simple as having the price drop to some particular amount. Still, as the price drops, the grandfathered shorts either have to realize more and more partial losses (and reduce their leverage), or they have to keep buying and adding more BTS collateral (further reducing their leverage).
I guess the only thing to get these guys to cover fully will be to either wait until September or perhaps if the price of BTS goes up high enough they might volunteer to cover so that they do not miss out on compounding gains.
By the way, FYI there are no short sells at the price feed again... I can't wait until this bear market ends.