To help figure out how much my PPM would change with the adding of nutes, I did a one gallon RO water test with each additive I had planned to use. For planning i used the recommended one gallon dosage, that way i know when i add that additive i am reasonably sure of what my new PPM number will be once i added with the top-off . You know the size of your res/bucket and you know your ppm/gal the rest is just math.
This works well for me as i do not change out the rez between grows & I keep my res at 64-65. I control the ppm by adding straight RO during daily top-off. I watch the ppm go down daily and when i reach my threshold, i add the nutes with the top-off based on the numbers. Sometimes i can add only one (usually bloom or grow food). I have to plan some of the other things (silica,
calmag,
hydroguard, +Life,) to be added when i can afford to not add grow/bloom food.
It works for me, hope it helps someone else.