1 gram of NaCl has 160 ppm chloride and 104 ppm sodium when disolved in 1 gallon of water. Since you only want 12 oz of liquid, 1 gram would give you a concentration of 104ppm x (128/12) = 1,109.333ppm. Too strong! So we solve for the reduced amount needed as follows:
50ppm / 1,109.333ppm = x gms / 1 grams
Solving for x gives .045 grams.