OK! First, In your receiver, there should be a menu which will allow you to set the size of your speakers, and the distance from your listening position (the "sweet spot") to each individual speaker, including the subwoofer.
There should be some master volume setting on your receiver that indicates it is a "reference" setting. My Onkyo has two volume scales. I can select the one I prefer. One is from -82 at the low end to +18 at the hight end. "0" is reference level. The other scale runs from 0 at the bottom to 100 at the top. The scale will read 80, then 81, then "Ref," then 83, 84, etc. Obviously 82, or "Ref," is reference level.
Your receiver should be set at reference level, And the SPL meter should be set to the 70 dB scale, C weighting, and "slow" response. Then your receiver should have some sort of a speaker setup menu that will allow you to set volumes for each individual speaker WITHOUT touching the master volume knob which you have set at reference level. It will send out a signal (pink noise) to each individual speaker, one at a time. Most receivers test signals are at 75 dB. Using the volume settings in the speaker setup menu, you adjust the volume of each individual speaker until the meter reads 75dB.
Here are some tutorials which might explain things better than I have.
A Quick Overview of Home Theater Calibration
Calibrating Your Home Theater System (Credit to BrotherBob for coming up with this one)
You may find these tutorials mention setup discs (Avia, Digital Video Essentials, other wise known as DVE, and Sound & Vision) While the test tones in your receiver will do the job just fine, I recommend picking up one of these discs. They have comprehensive instructions for their use, and include a lot of worthwhile info about video as well as audio. I have both Avia and DVE, and am glad to have both. Here is a tutorial which will introduce you to these discs and explain their differences.
AN INTRODUCTION TO TEST & CALIBRATION DVDS, aka "Why the hell do I need one of these things?"
