Ackman is smart guy and he would have covered his position by now. He will start shorting again in the $42 to $45 range again. After four to five round trips, he would have gained $20 per share five times over. So, best way to play this POS is to buy some calls for March at strike of $42 to $45, one contract for each 100 POS you want to short. When it gets there, which it eventually will sometime in the next few weeks, start shoring and Ackman will be beside you.
If it gets half way, just take the profits so far and walk away.