For my case i am not very particular about getting 75dB if setting the reciever's volume to 0.This is just a reference.what is important is the sound pressure level for all speakers must be equal.This is what i do,average all speakers gain and then use it as a reference.If you set reciever at -18dB and SPL meter is getting 78db,adjust all speaker gain to this value. The user manual and instruction on the autosetup screen indicated that the tripod must be placed on the centre of the seat for 3 point measurement