Super high combined chlorine

[chem geek]

If you want to know how much more chlorine it will take you can do a bucket test where 1/4 teaspoon of 6% bleach in 2 gallons of pool water is 10 ppm FC. That will tell you how much cumulative FC you have left to go. If one does this initially, then one can decide whether it's better to do a partial drain/refill if water is less expensive than the amount of chlorine required. One can also get an inexpensive ammonia test kit from a pet/fish/aquarium store to get some idea of the ammonia in the water (though that's only part of what needs to get oxidized).

As for adding CYA, just note that it will slow down the rate of the oxidation process. The plus side is that it will retain the FC longer during the day protecting it from breakdown from sunlight, but right now your FC seems to be mostly getting consumed oxidizing the ammonia in the water and that's much faster with no CYA in the water. Normally I recommend not adding CYA until your FC is holding longer. If the FC only drops by 50% in an hour, then that's the time to add CYA since that drop can be from sunlight. If it drops a lot more than that, then it may not be worth having the CYA. If you do add CYA, don't add very much -- even 10 ppm or 20 ppm is plenty at this point, but will slow down the oxidation process by over 10x. Finally, you'll know that sunlight and no CYA are not an issue yet if you add chlorine at night with the sun off the pool and still find the FC getting used up rapidly.

As far as your liner is concerned, just keep the pump running while doing this process and pour slowly over a return flow to ensure thorough mixing and lightly brush the side and bottom of the pool where you add chemicals. Your liner will be fine.

(On a side note, in a conversation with Ben I found out that he first discovered this effect long before I did and wrote about this here and here and in earlier posts. I just independently worked out some of the chemistry involved based on known CYA degradation pathways to come up with some low and high range quantities.)