Good luck with your fight about which way to maintain the pool
1.) Draining half your pool would get you down to 40, you could likely get by with a 25% drain that would get you down to 60.
2.) Regardless of what CYa you aim for, your day to day chlorine usage would not be a lot like you may be thinking. As long as your pool is currently clear, you don't need to do a SLAM, and you do mantain the appropriate levels of chlorine for whichever CYA you have you won't be using very much on a daily basis. Regardless of your CYA it will always take the same amount of chlorine/bleach to raise your FC levels by 1ppm. At 80 CYA you would be raiseing FC from 9 to 13, letting it fall till the next day and raising it again. At 40 CYa you would be doing the same thing just going from 5 to 9 instead but still using the same amount of chlorine to add 4 PPM
My suggestion, if your allowed.
Let the chlorine fall on it's own down to 9, your target level for 80 CYA. After that, add enough chlorine daily so that by the next time you test and add, it has not fallen below your minimum (6) I perrsonally try not to let it fall below my target level.
it will only take 32 oz of 12.5% chlorine, 40 oz of 10% chlorine, or 48oz of 8.25 bleach daily to raise your FC level by 4. I would be surprised if you ever lost more than 4 FC per day, and if it was more it wouldn't be bery much.
Of course, none of this matters if your parents win and their goal is to only keep FC at 1.