Ionic Nutrient Mixing

[FearAndLoathingInGR]

I am trying to understand the exactness of mixing nutrients and i am not finding the answers i am looking for.

 

I am using Ionics atm and according to the bottle ph should be 5.8 - 6.2, and the ppm is 1300.

 

This is what i need to know, i am using tap water till i move on to better things like RO, tap water runs approx 150 to 200 ppm. do i add that onto the 1300 for a total ppm of 1500? or should the bottle recommendation of 1300 ppm be the highest number you go to?

 

This may sound simple minded but i gotta ask anyway, i check my nutrients almost daily. lets say i do my rez flush on monday, fresh water fresh nutes, 1500 ppm. now when i check the plants on friday the ppm has not changed much. Are the plants not eating it up? Heres what i imagined, feed plants monday, by the end of the week i was anticipating the nutrient ppm to go down, it doesnt.

 

Thanks for the valued info.