How much do you add if you only have 640 gallon's and it states on the directions to add 4 Lbs to every 10000 gallons?

To find out how much to add for 640 gallons, first determine the proportion of 640 gallons to 10,000 gallons. Since 640 gallons is 0.064 of 10,000 gallons, you multiply 4 lbs by 0.064. This results in adding approximately 0.256 lbs to the 640 gallons.