Your method isn't a balanced one, if you are trying to match water chemistry. You need Magnesium chloride to go with the Magnesium sulfate you have. Just taking a cup of Epsoms Salt alone is unbalanced, and we know it is all about the ions.
Following RHF's Recipe #2, I use 7.25 cups of Mag Flake and .75 of a cup of Epsoms Salt carefully mixed with 1 gallon of RO/DI water.
I use Jose's calculator. If he needs to add Brightwell's additives to it, ask him.