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 ...
you need to drain and refill log0.95(0.5)=13.51 times. Water cost 67.5% of the pool water capacity
So if you can drain only once exactly as much as you need.
For me, 17% of pool capacity is about $30
Note: if you need to divide CYA by four and drain only 5% at at time, water cost is 1.35 times the pool water capacity - almost double the cost compared to draining once.