And I've never used the Pool Calculator. The formula for bleach is easy to use and there's even a short-cut, a rule of thumb so you can do it in your head:

1 gallon of bleach adds exactly its concentration to 10,000 gallons of water
So a gallon of 6% adds 6ppm of FC to 10,000 gallons
A gallon of 12.5% adds 12.5ppm of FC to 10,000 gallons
Therefore a gallon of 6% only adds 3ppm of FC to 20,000 gallons.

For everything else I usually start with a quarter of what I think I'll need. I can always add more, much more easily.

Carl