According to http://www.cag.uconn.edu/NUTSCI/NUTSCI/outrch/pdf/HowMuchSaltSugar.pdf there is 2,000 milligrams of salt in one teaspoon of salt. Using algebra you can determine that there is .36 teaspoons in 720 milligrams of salt.
Add 1/4 tsp of salt almost 1/8 tsp of salt together. You could add the full 1/8 tsp of salt but you will have just a little bit more salt than you need. It's not a big difference.
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