6.0 seems to be the number recommended most often. I am in my first coco grow and I am finding that as salts build up (very quickly) the PH drops out of the best range for calcium. Being just a few weeks into the grow, I am certainly no expert but watering closer to 6.4 is working better than 6.0. Just my rookie opinion, so don't take it to the bank. Welcome to the site. Semper Fi .