You use the same PoolMath, but due to rounding you might put in 10 times your spa volume and then use 1/10th the amount of chemicals it says to use. For example, increasing FC by 3 ppm in 350 gallons would say 0.3 ounces weight (0.2 ounces volume) of Dichlor, but if you used 3500 gallons it would say 2.5 ounces weight (2.4 ounces volume) so that you can see the real numbers for 350 gallons are 0.25 ounces weight (0.24 ounces volume).
You then need to multiply ounces volume by 6 to get teaspoons so 0.24*6 = 1.44 teaspoons so roughly 1-1/2 teasoons.
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