And at last, if you want to measure it, you can ! Send anything with a constant level in the input of your choice at -18 on a Logic track and then insert on that track a VU Meter plugin. If it says 0 dB, your audio interface is "calibrated" at -18 dBFS. If not, adjust the level until it says 0 on your VU Meter to find out.
So I did this (put gain plugin at beginning of strip) and increased it until I was averaging around 0 VU on the Waves VU meter (I have the "headroom set to 18 on VU meter). So 0 in Logic's fader meter = -18 VU. I thought that each logic track (to start a mix) should be around -18 / -15 dB in the fader meter, and have been leaving about -6dB headroom in my logic meters... that makes the VU meter come in around -5 VU... I'm more confused than I was when I started lol.