In general, you should use the C weighting and the fast setting on the SPL meter. What you should do first is establish a reference level with the two main speakers. It doesn't have to do with the volume position on the receiver, but with the actual output volume (although so long as you're consistent, it doesn't really matter). I typically use a reference level of 75 db, which is close to my normal listening volume, while others recommend going to 80 or 85 db as the reference level.

First thing to do is make sure that the L-R balance between the two main speakers is correct. Once you have those two channels equal, THEN you set the subwoofer, center, and surround speakers and adjust those levels until all channels are equal. The levels on those channels are designed to vary, whereas it's common for the L/R mains to not have a separate level adjustment available. That's why it's more practical to use the L/R speaker levels as the reference, rather than the center speaker.

Another thing to consider, the bass frequencies measure and sound a little differently than the higher frequencies because 1) human hearing is less sensitive in the lower frequencies; and 2) the RS SPL meter is less sensitive once you go below about 60 Hz. Because of this, a lot of people on this board recommend that you set the subwoofer level about 4 to 8 db higher than the mains (keep in mind that if your room acoustics create big bass peaks, then you will need to correct those problems before boosting the subwoofer level, otherwise the bass will sound way too boomy).

Also, I would suggest that you calibrate your system to the test tones on a calibration DVD like Avia, Digital Video Essentials, or the less expensive Sound & Vision Home Theater Setup disc. This is because the wideband test tones generated by most receivers include a lot of the bass range, and in rooms with acoustical issues, this can give you erroneous readings. The test tones on those calibration discs are narrow band and more accurate for what will actually play through your speakers.

Lastly, if you have a camera tripod, mount the SPL meter on it and angle it at about 45 degrees pointing towards the center (you should try and hold it that way even without a tripod). This ensures that the meter will consistently measure from the same height (which should be right at ear level), and that sound waves bouncing off your body don't give you erroneous readings. The tripod just gives you an added level of consistency and accuracy.