how you keep track of ppm

[fix]

Oh yeah, one other thing....I just asked about the magnesium. I'm wondering how you keep track of ppm by simply keeping track of how much nutrient solution you put in each gallon? Or does how many tsps. ,1/4tsps ,etc... that you put in each gallon replace ppm if you cant directly monitor it. I've read your horror stories about tds pens.... if you pay that money, it must be a worthy tool, right? If you cant tell, I'm a novice looking for some info, and all 32 pages here have helped me alot....THANKS.