I would NOT drain more than half your water if your CYA is 80. It will leave you with a CYA of 40 which is FINE for the summer. Check "Ben's Best Guess Table" and you'll see that at CYA 80 you need to shock your pool at 20ppm. At CYA=40 you'll shock at 15ppm.

Don't use their "shock". It will either raise your calcium levels far too high (Cal-Hypo) or lower you pH far too low (Di-Chlor)-you are currently at the best level for fighting algae at 7.2. Stick to bleach or Liquid Chlorine which are both pH neutral.

Don't worry: You'll get there!