I've read about using a large sheet of plastic to keep fill water separate from the water being drained - this way you mitigate against the possibility of floating your pool out of the ground.

You need a CYA measure for almost everything. Even to know how much water to replace.

Have you ordered a K-2006? You can also buy stand alone CYA kits, even at some pool stores if that's convenient.

CYA tests only measure to 100ppm but you can use the dilution method to measure higher. Mix 1 part pool water and 1 part tap water (tap water has no CYA) test the mix and multiply the result by 2. If it's still over 100, dilute 1 part pool water with 2 parts tap water and multiply by 3. This will give you a range of how much to drain / refill.