ppM question.. need advice

Ok guys and gals I have been wondering about this since I started growing a bit over a year ago. My tap water goes straight into buckets and get airstoned for a couple days before watering. Once a couple days go by I test the water and usually comes in around 190-210 ppm. According to my tester. Now I want to feed seedling about 175-200 ppm. Does that mean I can add 200ppm to my water which ends up being around 400 ppm all in. Someone set me straight.

I’ve heard people say this before and it makes no sense to me. The PPM of the solution changes the way the plant can absorb the nutrients, so I would think that you should be using the sum total ppm of the water and waterer you add to it. When I needed under 100ppm I would mix RO water in until I hit where I needed to be. I could be wrong though...but it’s how I’ve always done it.