Figure 15,000 gallons for your calculations...if you have 4' of water the actual number is about 14,700 gallons + or - 14 gallons. But nothing's that accurate and 15,000 is MORE than good enough--makes rule-of-thumb and ballpark calculations easy.
1.5 gallons of 6% bleach adds 6ppm of chlorine to your water. 1 gallon only adds 4ppm. If you buy 12.5% liquid chlorine at a pool store it should add 8.5 ppm of chlorine per gallon.

Carl