I don't have the SPL meter you do I have a Radio Shack analog meter but the principle is the same. First you should measure from the "sweet spot" in your room. If your single this is where you usually sit. If married where your wife usually sits.

I'm not sure if you have a 990 yet but you would go into your pre/pro or receiver's setup menu and go to "calibrate speakers" or "speaker level". You'll need to know the reference level for your receiver or pre/pro. You set the meter to the desired db level, usually 70 or 80, and then you hold or place the meter in the sweet spot, preferably at listening position, set your volume at the reference level and run through the test tone on each channel setting them as close to "zero" as possible.

I don't give instructions very well and I'm sure someone here can be more precise but that's the general idea.