The amount of FC to get rid of CC (if it's monochloramine) is only somewhat more than half the CC level. The 10x rule is wrong when applied to CC. If you measure ammonia levels, then it's at least 7.6 and normally 8-10x that ammonia-nitrogen level as FC to get rid of it. On top of that, there will be partially broken down CYA that doesn't measure as ammonia nor as CC.

Basically, just blast it with a lot of chlorine. If you want to predict how much will be needed, then do a bucket test and note that 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC.