Okay, here's the way I look at it:
0 dBfs is not the same thing a 0 dB on a VU meter in the analog world. 0 in the analog world is usually a nominal level that you can feel free to go over. 0 dBfs is an outside limit that you can't go over.
0 dBfs is an immutable level (if you will), at least given a particular recorder or converter. It's the point at which all bits are on. If you try to run peaks past it, you will flat-top your signal, producing fairly obvious distortion. The voltage at the input jacks that produces 0 dBfs depends on the design of the recorder or converter, but it's pretty much what it is.
If you're just thinking about calibrating a VU meter so you can use it in a way you're accustomed to while recording to the digital recorder, it's sort of a matter of your own preference how to set it. If you're used to running way into the red (like, say you're used to hitting analog tape really hard), calibrate 0 dB on your VU meter to -20 dBfs. If you're accustomed to being really conservative (a recorder of classical music, perhaps), you might calibrate the VU meter to -6 dBfs.
