Your base water EC when running RO water + calmag should be 300. Then you add the NPK type nutes on top of that.
Your water may technically be considered 'hard water', and there is I believe a selection of hydro nutes designed to be used with hard water- you'll want to google 'hard water hydro nutrient' for more info. If you can't get the hard water nute formula, or RO water, you might consider doing a soil grow.