Yes, you've used the Rule of Thumb in reverse, and done so correctly. It's not that accurate--you are still fighting bio mass, and an elevated CC. You also don't know if the 12.5% is actually that (you can test it, easily enough) So a gallon of 12.5% raising it 6ppm doesn't guarantee your pool is 21k. It could be as low as 18k or as high as 22K. But the good news is: It doesn't really matter. Just assume 20,000 gallons and figure accordingly. You'll be fine.

To test your LC's strength: you'll need a 10ml measure--Drug stores sell glass eyedroppers marked to 10 ml. Fill a 5 gallon bucket with 10 liters of tap water....that's 5 x 2-liter soda bottles. Test the tap water for FC and CC...if it's city water you'll want to know how much is there and subtract it. Now add 10ml of your 12.5% to the 10 liters and mix. Test it with your FAS-DPD test. Whatever the test shows, minus the chlorine that was in the tap water, is the actual strength of your LC. Mine usually tests at 14ppm, but it may well be as low as 10ppm.

Again, 20,000 gallons is good enough for your reckoning.

Carl