You have an excellent resource for just this question in Texas. Unfortunately it will cost you a soil test to get the info you want.
Talk to K Chandler at
http://www.txplant-soillab.com/
Tell him about your water, your soil, and your grass. He may want a water sample, too. His soil test will tell you exactly how much magnesium and calcium you need to balance out the salt. In Texas hardly anyone needs calcium, so it would be just magnesium (Epsom salts). He specializes in getting the salt balance right.
His soil tests are slightly more expensive than those found elsewhere, but if you asked the other places to do all the testing K does, not only will it cost a lot more, but they won't know how to interpret the results or be able to compare yours to the thousands of other tests he's done. K is well known as THE premier soil tester for organic applications in Texas if not elsewhere.