I've always used the 12,000-15,000 gallon estimate because it's what the pool store guys told me (they open and close for me) and when I've posted various things here, some of you agreed. Today I did a FC test, I was at 5.5 FC. I used 15,000 gallons in pool math and added 69 oz chlorine to reach 10 FC. 15 min later (also very cloudy day) I'm at 10.5. So, 15,000 is pretty close, but I'm not good with this kind of thing, can someone tell me closer what it is? Since it was higher am I more like 14,000?