I don't know much about the SWCGs, so I'll let some one else answer that. How is your pool water being tested? Pool Store, guess strips, oto/phenol red, FAS-DPD?

I can tell you that with a CYA level of 70, you want to keep you chlorine ABOVE 5ppm at ALL times. This explains why. http://pool9.net/cl-cya/