Before you do a partial drain and refill, you might want to figure out just how high your CYA actually is. The limitations of the test will not differentiate readings higher than 100 so a result of 100 might mean 100, or 300 or 500 or ?? If it's super high, then draining a third of the water might not make that much difference.

Dilute the water sample one part pool water and one part distilled water and then run the test again and multiply the result by 2. If you still get a reading of 100, do one part pool water and two parts distilled water and multiply the result by 3, etc.