If you add a known amount of some chemical to your pool, and measure the levels before and after, then you can calculate how many gallons. The most accurate test is for chlorine, so usually you'd use bleach.
Now, 1 gallon of 6% bleach in 10,000 gallons will raise FC 6ppm.
Therefore, a 182 oz jug (1.4 gallons) in 14,000 gallons will raise FC 6ppm.
For this procedure, you probably want to use a 25ml sample for your FAS-DPD test, to get the 0.2ppm accuracy. What you do is:
1) Make sure you don't have any algae or other organics using up chlorine.
2) Wait for the sun to go down (another source of FC loss).
3) Run the FAS-DPD test with a 25ml sample.
4) Add your 182-oz jug of Clorox 6%.
5) Let the pump run for an hour or two.
6) Repeat the FAS-DPD test with a 25ml sample.
7) Take the difference of the two FC readings.
Now calculate your pool capacity = 14,000 x (6 / FCdiff).
--paulr