Using Cal-Mag Plus at 5ml per Gallon will add around 90ppm to 120ppm to your water. Around 40ppm of calcium, 30ppm of nitrates, and 20ppm of magnesium. Most nutrients are designed for tap water with 150ppm to 200ppm, usually containing a variety of minerals in low concentration with a good deal of Cal-Mag already.
Personally, my tap water is soft, coming in around 60ppm. When it gets broken down, it is really very little calcium. This requires that I add about 5ml per gallon EVERY time I add water so I can get my tap water up to a level that my nutrient brand is designed for, and to prevent deficiency.
Without knowing what is in your water already you're really blind. The fact that your water is acidic (while most are more alkaline) actually indicates that it may not be the highest quality. Perhaps with very high levels of chloramines.
Your water company should be able to provide you with a report of your water quality free of charge. These usually contain readings in ppm that should let you know, even without a meter, about what your water has in it. Then you can adjust your Cal-Mag usage based on that.