Pool Calculator; useful?

[JimK]

Some use it and swear by it. I never do.
I figure 1 gallon of <n>% bleach in 10,000 gallons of water raises FC by <n>ppm. It's a convenient rule of thumb based on the actual formula: (1,000,000/pool vol) * (bleach concentration) * (amount of bleach) = FC increase.

For those of us who are more mathmatically challenged, could you give an example (perhaps using my 20k gal pool using 6% bleach)?

Thanks.