1 Gram of Epsom salt in 1 gallon of water will provide 26 PPM of Magnesium and 34 PPM sulfur or 60 PPM total.
You could grab a scale and weigh a tablespoon, then multiply to see how many PPM the mix would be. (Elemental PPM)
I personally wouldn't go past 150 PPM total (Magnesium and Sulfur). 2 grams per gallon would be what personally consider to be a "strong" foliar feed at 120 PPM.