Here is a quote from the below article on Audioholics.
"Step 7 Sound Pressure Level (SPL) Calibration
Place the SPL Meter at the listening position at ear level with the Mic end pointed toward the ceiling. Select “C” weighting, response slow then, turn the dB dial to 70. Activate the internal pink noise generator of your Receiver and select manual test tone. Now adjust each speaker to +75 dB reference. When you are adjusting the surrounds make sure your body is not in a direct path of the speaker and make sure the house is quiet. When doing the calibration, only the test tone should be heard (A/C, ceiling fans, ect. should be turned off). I like to adjust the subwoofer level to + 80dB for wow effect. Your taste may vary."
http://www.audioholics.com/techtips/setup/loudspeakers/loudspeakerplacement01.php 
