don't check your ppm of runoff cuz its only makes things complicated. If you need to find out the ph or ppm of your soil or coco you simply have to do a slurry mixture. This is done in a 1 to 1 ratio and what that means is that you dig about 2inch into your media, and then you grab a half a cup of your media and the other half should be distilled water. make sure that your mix is soupy. You wanna mix this for 5 min throughout the day. At the end of the night before going to bed mix it one more time and than in the morning the mixture has settled and you should have somewhat clear water.
That's how you check the actual ph and ppm of your media.
You should always know what ph and ppm you are putting into your media. It will reduce problems