And what do these details add up to?

It still comes down to putting enough chlorine into your pool to make it sanitary. Is it going to matter if you think one gallon of LC is going to add 10.8ppm to 10,000 gallons when it only adds 10.3? What really matters is if the stuff is still good and relatively close to what it's supposed to be.

I'll bet not one person in 100 knows within 200 gallons how much water is in their pool. And if it's anything bigger than a donut, I'd guess that variation would be 500 gallons. I'll bet nobody here knows it that accurately. We know APPROXIMATELY how much water is in our pool. And, when it comes to shocking a pool, it doesn't REALLY matter if the pool is 19,000 gallons, 20,000 gallons, or 21,000 gallons. You can safely use 20,000 gallons for all your calculations.

So...where does the problem with this measure of concentration crop up? Where is the bleach calculator formula wrong? Why is it wrong? More importantly, is it wrong enough to matter? Or is it a good enough approximation?
(we know Isaac Newton's laws of Physics are fundamentally wrong and that Einstein's Relativity fixes those problems, but Newton's physics is STILL a good enough approximation to use to fly a probe past Pluto without resorting to Relativity. See my point?)

More importantly: Can YOU come up with a better method of determining if your bleach / liquid chlorine is still good?
By better I mean something a homeowner can practically implement, not something that needs a sterile lab with people in white coats.

Carl