Here is a link to a great paper on a study of CYA.
http://www.ppoa.org/pdfs/PrP_Cyanurics% ... 20Bomb.pdf
the upshot is, keep it as low as you can.
Anything over 70ppm, and you can throw TONS of money away in chlorine that will not be effective.
After reading all this, please just do as much of a water change as you can afford (100%)? and stop banging your head against a flagstone.