When you are PHing water, you are PHing the actual mix you are feeding them.... So, you want to take your Coco A+B and add it to the tap water, and mix it all up.... Once your done, you take your PH meter and take a reading..... Your growing in Coco, so the PH is different from Soil... YOu want the PH of your feed going in to be between 5.5-5.8 It probably will be like 6.6 after you mix it up, so you will need to add PH DOWN.... Add it slowly though... its strong... 1 ML will drop a gallon of water a whole point.... Just add a little, mix it, take another reading.... Once it is between 5.5-5.8, it is safe to feed your girls....
As for Runoff PH in coco..... Your Runoff to waste water should have a PH of @ 6.0 If it is high (anything over 6.3), you need to Flush immediately with Properly PH'd Water ONLY..... I believe you are gonna have a REALLY high PH level if you havent been PHing your feeds... When you feed the girls tonight or tommorow, Make sure you PH the water going in correctly, then take a sample pf the runoff, and let me know what it is.... I had this problem a couple months ago....