measuring PPM's in COCO???

HI kind people!
This question may seem outrageous to ya'll but i am going to ask anyways. I am totally new at all this and have been experiencing all the problems and lessons that a noob should (could).

I have some girls going in soil right now and seperate girls in coco. They are all at about 2 wks bloom.

I went out and got a PH and PPM meter so that i would not be in the dark.

Does it matter what the PPM already in the COCo is when decidin g how much to feed? I was checking the PPM of the runoff and finding it about 600 ppms. This was when i was just putting my plain 100 ppm well water through it. The water was coming out the bottom at 600 ppm. When deciding how much to water them do i figure this nuymber into the equation?
I have looked around and tried to find an answer but could not. Can anyone help?

Also i found the PH of the coco runoff to be about 6.3. I know it would be better at about 5.8 so when i feed next i will drop the PH in the feed to about 5.7 to try and fix this. Does that sound like the proper thing to do?

THANKS BE To ALL YOU!!!

allrollsin21