yes, you are think the wrong way
what you need is band limited pink, pink nose with HP at 500 and LP at 2000 (aka Dolby Pink) this helps eliminate room modes from effecting the measurement level
so 1 channel set to 1 output, SG set to -20dB Pink, EQ3 with LPF set to 500 & HPF set to 2k
measure the SPL for that channel, trim to 85 dB slow C
change the output for the next speaker
repeat
setting the SG to 0 would make you calibration 20 dB to low / you mixes would be ~20 dB low