1) drain 1/2 the water (in one shot) and refill. Cost: 50% of the pool water capacity
2) drain 1/4, refill (now CYA is 3/4), drain (1/4) refill (now CYA is 0.56), drain (12%) and refill. Cost: 62% of pool water capacity
3) drain 5% refill (now CYA is 95%), drain 5% refill (now CYA is 90%), etc...