(Water PPM + Nutrient PPM = Correct PPM) OR (Total PPM - Water PPM = Correct PPM)

This is probably digging in the weeds too much and I'm sure a very newbie question, but what is the correct way to ensure enough nutrients are in your water?

I have an RO system that is turning out approx 50PPM H2O. I'm just starting my 3 week olds on a feed schedule and it calls for 400PPM for the first week. Do I add nutrients to the RO water until it hits 400? Or do I go to 450, taking into account the PPM of the RO water?

Add 400 ppm of nutes - your total ppm will be 450 with your water.