PAR stands for Photosynthetically Active Radiation i.e. light available for photosynthesis. As you go down more in the water column, the light will be more diffuse, and the PAR will be lower since it is traveling through more matter to reach lower areas of the tank. Therefore, corals requiring higher PAR should be placed higher up and in the center. Individual PAR values depend on the type of fixture and intensity settings. The best way is to see if your local reef club has one that they lend out. Otherwise, I know that BRS sells the Seneye which can measure PAR as well as other parameters if I'm not mistaken.