I shoot for 50ppm per gallon on the tds meter. What are you using it for?
I use it to get 7ppm of elemental sulfur per gallon, the Mg is a non-issue for me.
I suggest you take about a pound of MgSO4 (500g by weight is a good start) and dissolve it in a gallon of water to make a stock solution, decide what your target MgSO4 ppm is and why, then take some measurements and go from there. It depends what you have going on already. I use MgSO4 from week 2 to week 7, only for the sulfur, because otherwise I don’t have much S in my solution at all. Media, water source and fertilizers you are already using are important factors in this decision.
Do you have a TDS meter? What country did you buy it in? (Seriously) Are you in soil? What are your water source parameters? Fertigation plan? It all matters.