The limitation is the bandwidth of the microphone element, it is only efficient down to about 60Hz. Below 60Hz it rolls off very steeply..
I wouldn't worry so much about an SPL measurement for the sub, but be more concerned with a full bandwidth response without the bass being too boomy... Set the other loudspeakers @ a balanced SPL level, set the subwoofer level on the box itself to about 50-60% rotation. Next starting with the Subwoofer's level set @ 0dB in the AVR, you should only have to make minor tweaks..
Also physical positioning of the subwoofer within the listening room can greatly affect the bass, distance from the wall or being in a corner delivering the most bass.
Keep in mind, that the sub level can/will vary greatly with various source materials with music or cinema tracks.. Plus personal taste of the listener..
Just my $0.02..