-6dB is a good start, Having said this, you can creep up; however, leave it at -6 until you get a beter grasp. (My recommendation as mentioned earlier: -6 to -3dB at the most. If you can increase/decrease by .1dB then, do so as well. That small an increment makes a difference)
No. Different VU Meters (digital/analog) have different values, so the increments may throw you off. ie: If the meter reaches -6dB or above, you'll clip. Likewise, if it says -3 or 0. However, many LED digital meters are color-coded. Green/Yellow/Orange/Red.
Obviously, if any red is showing, you are clipping; if it's staying red, well ...
Now, sometimes, it'll reach red, but if it taps, it's okay; yet, only if this can't be help. For instance, the kick on a drumset. However, if there are low end like instruments with the kick, then this may be a problem, but it can be fix. Tell the drummer not to kick so hard or have the mixer person adjust accordingly. However, it's accepted for the kick to knock-in-the-red; however, you don't want it killing the mix because okay doesn't mean "as hot as possible"
Recording beginners will make the mistake of recording hot-tracks. Do not do this!
As Danny said (and I'm still waiting on him) - "if it goes in bad, it'll come out bad." If it goes in loud, it'll come out the same way. Get your source right.
More often, than not many meters will display 0.0 as the no-clipping limit. Pay attention to the peak meters - not just the solid/thick LED lights. Since I can't post a pic:
the i's (peak decibel) have it. The l's (decibel indicator) do not. Don't ignore the colored lights though.
Be very aware though that you can't always trust them, so use the other volume visual aid tools at your disposal.
Remember, you want headroom, even if there's still a little bit after you're done mastering, but most definitely when you're recording/mixing.