It can be placed anywhere with a couple of concerns. The Pinpoint can be susceptible to the excessive emissions from certain reef lighting systems. So you may get odd readings when the lights are on if it is too close to a light or a ballast. Sometimes battery works better than AC as interference can also go through the AC.
Also, if the flow is low enough, you can get a slightly different pH in a refugium than in the tank. You can always check that.
I recommend 2.5-4 meq/L (7-11 dKH) for any tank where you have corals or coralline algae.
