I use PPM as I think it's a lot more accurate than EC, either way it's pretty simple, most PPM conversions to EC are on the 0.5 scale so 250ppm = 0.5EC, 500ppm = 1.0 EC.
It's pretty simple, as hashy said if the TDS of your run off is higher than what you are putting in, you got a build up in your media, and or plants are using more water than nutes, and could use a flush. If the TDS of the run off is lower than what you're putting in, then you can up the nute strength. I don't know why people say not to check this, it's very useful information to have, I check the run off TDS all the time, rarely bother checking run off pH in my coco run to waste setup.
In my current grow I let my PPM get as high as 950-1000 during weekds 4-5 while adding PK but mostly maintained it around 800 which is 1.6 EC.
Hope this helps.