Much like a normal chain, there isn't one correct answer. So long as the belt stays in place and does not slip, there is a wide range of acceptable tension. With too little tension, the belt will skip; too much tension creates noise and is harder on bearings. The best choice is somewhere in the middle.
You can download an app for Android phones to tension it according to sound, but I haven't found the app to work very well. I prefer to just do it by feel, looking for about 1/8" to 3/16" deflection at the belt.
You can download the app there, and if you use it, you want to aim for a value between 35-50 Hz.