the reason for that is; If you want to be at a certain ppm value, then this is advisable. You see additives, have there own ppm values, some more than others...So, if you limit yourself to say 1200ppm and want to add calmag, Liquid karma, and big bud...Mix them in your water, then you can take your reading, make up the difference between that and your desired ppm's, by adding your 3-part mix. peace