I use a simpler but related method, that's only good for add bleach/liquid chlorine, that I call The Rule of Thumb:

1 gallon of 6% bleach adds 6ppm of FC to 10,000 gallons of water.
1 gallon of 5.25% bleach adds 5.25ppm of FC to 10,000 gallons of water.
1 gallon of 12.5% liquid chlorine adds 12.5 ppm to 10,000 gallons of water.

If you have 20,000 gallons, then you need 2 gallons to get the same effect. If you only use 1 gallon you'll get half as much FC.
If you have 5,000 gallons you need half as much (2 quarts) to get the same effect. If you use 1 gallon you'll get double the FC.

So, if you have 30,000 gallons, you need 3 gallons to get the effect of 1 gallon in 10,000 gallons.
If you have 15,000 gallons, you need 1.5 gallons (or 6 quarts) to get the effect of 1 gallon in 10,000 gallons.

If your pool in 13,000 gallons, you can use the 15,000 gallon approximation, etc.

NOTE: This does not work for dry chlorine forms like dichlor, tri-chlor and cal-hypo.

Carl