Okay, as you know I am stupid, so just to clarify.
I don't understand what happens when, I add ph down, my water is at around 250ppm out of the tap and at 8.5ph, after ph'ing it down it is almost 300ppm! so if I want to be at 400 total, I can only add 100ppm with nutes, (doesn't seem like much?) or do I just add the 400ppm with the nutes in addition to what it starts at?

And then once it is adjusted, do I need to account for the ppm change from adding the ph down?
In a few days my ppm will be higher than where they started, but it's just the ph down, will it keep the plants from feeding on the nutes? I will go away after this and bother someone else for a while I promise! Thanks WH.