You're using poolmath correctly, but something doesn't compute.
People adding jugs and jugs are trying to get to shock level, often in the 30s. And they may have a much larger pool. If your pool is truly only 12,000 gallons, then you should be seeing the doses poolmath is telling you. But you should also be seeing some results. If you added 6 FC this morning and you're down to 1 FC now, that's a lot! About double what I'd expect to see.
So... perhaps your pool is larger than you think. My pool is basically a 12 X 32 kidney and 3 to 8 feet deep and it holds 16000 gallons. Second, the 10% bleach could be old and weak. Third: you have something growing in the water that's using your bleach.
How does the water look? Is it clear? Can you easily see the drain in the deep end? Any patches of algae?
The bleach: do you still have the cardboard boxes? It should have a date code somewhere. Do the boxes look old and shelf-worn, like they've been sitting out in the patio section at Lowes since last year?
You might just have to run an
Overnight Chlorine Loss Test to determine just how high the bleach is taking the FC in reality and how much you're losing. Calculate the dose to get to 10 or so, maybe a jug and a half. Let it circulate for at least an hour while you make a lap around the pool with the pool brush to really get some cross-currents going. It's like stirring milk into your coffee, just on a gigantic scale.
After the sun stops shining on the pool, go test the FC. Remember that number. Set your alarm clock and get up and let the water circulate again and then test FC before the sun hits the water. If you lost more than 1 FC overnight, you have a problem. Only two things destroy FC: UV/sunlight and organic matter. Since you've eliminated light, it will have to be matter.