Just draining 12"? Sorry, overlooked that.

Assuming your pool is all shallow (5' max depth) you will still only lower CYA from 120 to 90 -- still VERY high, especially for a screened pool. Did you read the Best Guess page? You really need to do so. Chlorine is not suddenly 'locked up' by stabilizer. A more accurate description would be that an ever increasing portion of the chlorine is 'placed on reserve' as CYA levels rise, and only 'recalled to duty' as HOCl level drops. You can operate a pool successfully with a CYA of 90, but you will have to follow the Best Guess table to do so . . . and you must stop using stabilized chlorine as your chlorine source.

The good news is, in Florida it's often possible to buy commercial bleach ("liquid chlorine") very cheaply. Other wise, you'll need to buy gallons of Walmart's 8.25% bleach.

Cal hypo is also an option, but only really practical if you have a sand filter.

Post your K2006 CYA test results, ASAP.