This isn't a ph issue, you're doing fine with your watering. 12 inches from a 1000 watt hps is a bit close! You can see some slight leaf cupping at your top leaves. The necrosis you're seeing is a magnesium deficiency. By the time it gets to the brown burning like that..it's been there for weeks. It starts with yellowing between the veins, and you can see it on the lower leaves in your pic. It gets burnt at the top first because the demand is extra high there, especially being so close to a powerful light, and the plant cant keep up with the demand fast enough..especially in mid-flower!
Did you wash/buffer your coco prior to use? What and how much do you feed and how often? Source of water? For now i'd say add some extra cal-mag, or add epsom salt to your waterings.