Since you reported such a big loss during the day on one of your recent posts, keep your chlorine at shock level for a little while longer. (With a CYA of 80 or 90, you wouldn't expect to see such a large drop in FC in one day)
Once you are in maintenance mode instead of shocking mode, you'll want to keep your chlorine between 5-10 all the time.
As far as the pH test, with a K-2006 kit, I think as long as the FC is below 15, your pH reading should be accurate. (With other kits, the FC needs to be below 5.)