^^ if your receiver calibrated everything correctly, measuring in REW with -3db FS at 0MV will have your FR sitting around 115db for lfe channel and 105db for speaker. If you want to verify your absolute level accuracy and don't want to be loud, set your MV at -30 and measure with -3db FS in REW should yield your FR around 85db for lfe and 75db for speaker. As of starting measuring at 18hz, 1hz below tune is to see if you have any room gain below tune and to see where it actually rolls off in your room. My FV tune is 12hz but my f3 point in my room is down to 10hz.
Edit: 0MV on Denon Avr is reference. Not sure about Yamaha and others.