I am using a digital salt meter. Pool started out at 2,475 ppm and adding 120 pounds of salt got me to 2,800 ppm. If I try to use the Pentair salt calculator and work backwards from there, then I get something around 40,000+ gallons.

Just wondering if there is an algebraic formula for this calculation. Actually, I know there has to be one that these online calculators use, but my Google fu is failing me and I cannot find the formula. Not only that, but my algebra skills from 30+ years ago are failing me in trying to create the formula on my own under the assumption that 1 pound of salt increases salinity by 3200 ppm in 37.5 gallons of water (got this from my Google fu and cannot translate it into an equation for my needs).