You can if you want do it the way Stinky suggested, most gardeners do. However, you should try doing it in reverse (almost) order.

RO your water (don't worry about ppm) and add your base nutrients, your micro and your calmag. THEN check your water's pH. Without touching your pH up or down you should be right on. If you are off then use a little up or down. Try using as little as possible, its not good for your water.