The acid does not eat up the chlorine...if you've had Cl of .5 with a CYA of 40-50, then chances are you've got something blooming in the water that's causing the Cl demand. With a CYA that high, you need to always keep it above 3. I would try shocking the pool, let the cl drift back down to the 4-5 range, and then figure your consumption from there.

Janet