PH placement can take a lot of trial and error to get right. I prefer a chaotic flow (best for coral) which can he achieved by pointing PH's at each other. It is difficult to find the optimum position that leaves no dead spots in the tank. I find having them low pointing up towards the surface works best. I have both of mine in the back of the tank hidden behind my rock. It took a lot of small adjustments at least for me to get it just right.
In a 55 gallon (48" long 23" deep) tank I have one about 5" above sand bed and one about 10" above sand bed. The one that's 10" up is slightly more powerful then the other and gets good water movement on the surface which is also important. This is just what works for me based on where I have certain corals in my tank.