Draining the pool is a very tricky business on liner pools. One best left to pros.

You can use dilution to measure CYA higher than 100: Mix 1 part pool water and 1 part tap water (tap water has no CYA), measure the mix for CYA and multiple by 2. If that's still too high to read (mix measures over 100) mix 1 part pool water to 2 parts tap, measure and multiply by 3.