Josh, apparently no one HERE has purchased an SUV1 for an XL1, I've asked Beachtek many times for a review unit for the mags I write for. They don't even respond to my mails nor my requests in person at trade shows. So....can't help you.
What I can tell you is that if the unit won't go to -12, let alone -20 to be calibrated in tandem with the camera, then it's a waste of money and I'd be truckin' back to the store with it.
If Harry at Beachtek can't tell you whether the displays are calibrated to FS or VU, and tells you to set it to zero, I sincerely hope that you are only misunderstanding him rather than him not knowing the difference. That would scare the heck outta me. and would surprise me too. (him not knowing)
[edit] I've just downloaded the brochure and looked at the specs. It has a -21dB low end adjustment value according to the book, so it should be able to match what you have on the cam.
Print a 1K tone at -12 from your NLE to a tape. Put the tape in your cam. Play it. Use the jewelers screwdriver to adjust to match the output levels also being shown on your XL1. This should be all you need to do. If the brochure is wrong, then I'd be surprised as it appears to be fairly standard.[edit]

All I'm saying, and the last I'll say (I think, you never know), is this: If I set 0 on the Camera's VUs to 0 on the XL1s' VUs, then the problem is

1) when and if the camera's VUs peaks, and the audio distorts, I will simply think, looking at the SVU (yes I'll also have headphones, but that's not the issue here), that I've simply gotten to the bare limit of what's acceptable, not that it's actually distorting, since the readout on the SVU won't push past 0 and into the +3 and +6 range.

2) Since the lowest number on the SVU is -21, if I record anything lower than that (say, background noise, whatever), it'll look like I'm not recording anything at all, since the SVU won't read any lower.

These two things would seem to suggest that unit is not meant to be calibrated to my camera's 0, but there doesn't seem to be any other way to make it work.