I use botanicare calmag+ 1ml/gal no matter what with Reverse Osmosis. Think of it basically you're taking a completely unbuffered filtered h2o and then tweaking in supplements to get it where you want it without the bad stuff. I think of it as I'm basically taking my shitty well water, turning it into pure reverse osmosis then I'm supplementing it with calcium, magnesium, iron among other minerals.
Always add calmag after your base nutrients. Silica > Base > Calmag > Supplements/Stimulants