With a CYA of 0, you need to keep your chlorine up there between 12-15 by testing and adding more chlorine as many times as possible during the day to maintain the 12-15 ppm. I personally would switch back to plain, unscented bleach--your calcium levels are going to rise substantially if you're using cal-hypo to shock, and once they get to 400 or so, you're going to start having milky water issues. Keep it high until the pool clears, and your CC goes to zero.

Keep the filter running 24/7 , brush the pool daily, watch your filter pressure and clean it as necessary.

When you had a CYA of 40, was that earlier this season? Or at the end of last season?

Edit: Looks like Watermom and I were posting at the same time....but the advice is the same!