I've been adding nutes on TOP of my tap water, if I had a tap water with a TDS of 200 and wanted nutes at 400 I would add 400PPM worth of nutes into the res.

If I used RO water with a TDS of 50 I would add 400ppm of nutes

Maybe I've been doing it wrong but thats how i've been doing it and it seems to work great,

but im only on my first grow so dint listen to me too seriously