I have very hard tap water, high PH and "high" acidic capacity due to a lot Ca (hope thats the correct en expression), so I have to use RO Water, but I mix it on purpose with tap to get a base of Ca in.
Get a water analysis chart of your tap.
My water is N "heavy" as well but it's nearly free of chlorine or other bad things (tap water is controlled under food law where I live, so nearly no treatment needed, but for high Ca - like high Ca spring-water - I drink my tap water all day), But to my experience I can leave that N out of the calculation when I mix.
RO=3/4 Tap 1/4 coming to an EC of 150, ppm 75.
Calculate Ca and N, look if/how it fits to your feeding scedule. Add Epsom salt only, ditch calmag.
Check whats in your water, and decide on that.
Cheers!