Anyways..... Bert add the calmag and any other additives first manufacturers instructions( low end in my experience) then bring your ppm up to target with your base nutes. Don't put your ph tester in pure ro water it will fuck it up. Ro water has no potential hydrogen, it takes on the ph of the impuries you are adding(sort of) ie nutes. Now, if you get defficencies next run with ro or ph is unstable, try adding back about 20 % waste water. I know, I know contamInate your precious ro.... But cannabis needs other micronutrients that are naturally in tap. Rather than trying to chase defficencies you put a little of everything back in the solution. Helps buffer the ro as well.
This is what my hydro guy suggested aparantly aquarium people do this as well. IMHO 300 ppm tap should be ok without ro filteration( you did say your tap was 300 didn't you?) but it's your pesos not mine. My ro unit is somewhere in the shed. I'm back to tap as I can't justify all the waste water and time farting about with it. I did a couple of ionic runs after your last thread. Totally underated stuff man, as good as anything else out there.
Read as much as you can about ro water, it can throw you a curve if you ain't careful.
Ttfn
Subzero