I do it manually one of two ways:

Formal:

(1,000,000/pool gallons) * bleach concentration * Gallons of bleach = FC added to pool
Bleach concentration--if it's 6% bleach use .06, if 12.5%, use .125, if 5.25%, use .0525, etc.

If you use liters then use liters in BOTH places as Pool Liters and Liters of Bleach and it works the same.

Informal:
Or use the simple Rule of Thumb:

1 gallon adds exactly its percentage t0 10,000 gallons of water.

So... 1 gallon of 6% adds 6ppm to 10,000 gallons.

If you have 20,000 gallons, then 1 gallon of 6% only adds 3 ppm of FC.

1 gallon of 12.5% adds 12.5ppm to 10,000 gallons.

3 quarts of 6% adds only 4.5ppm to 10,000 gallons.

Adjust accordingly, but I can usually do it in my head this way.