I use R/O water so I have a starting value of 0 ppm on my water. but my readings were always close to the desired ppm readings listed. I would assume you account for any dissolved solids in your water by subtracting it from the total of the reading you get after adding your nutes. However I only took readings during my very first run using the formula. I realized that it worked and I didn't need to be caught up with the ppm readings. AS long as you follow the formula it works. I topped my res every couple of days but I never needed to adjust the pH.