Calibrating Your Audio With an SPL Meter

[SPIDERone]

Thank you!
Last question -- I have 3 subwoofers. 2 Front L/R and 1 Rear center. How would I calibrate the levels with this? Would I calibrate them one-by-one to 63DB (when the speakers are 60DB?), It seems like that might give me much more than I'm wanting/needing. Would I calibrate it with them all "on" until it reaches 63 DB? (the problem there is, what do I set each level at?)

Since I only have 1 SW Volume on the receiver, I'd need to control each sub beyond that by using the gain. I'm not sure if I should match the gain on each (ie: 50%) or how to tell which should be at 50% and which should be at 30% (for example).