[Rich Breen]

A few things to note here - some of which have been touched on by others:

You will never get close to most mastered levels without significant amounts of peak limiting - this is generally best done in the mastering phase *after* the mix has been printed with reasonable amounts of headroom.

Displayed waveform height is completely variable by the user, and trying to set levels by it is a waste of time, but ctl-opt-cmd-[ will return vertical zoom to default.

Trying to set loudness by VU meter is difficult for a novice; better to use a loudness meter (LUFS/LKFS) - nothing can beat your ears for determining loudness, but these will give you a better idea of actual loudness level than a VU meter. Peak meters are good for determining available headroom.

The best method for managing loudness is to work on a *calibrated* monitoring system; if you know where -20dBFS RMS = 85dB SPLC Slow is (or whatever level is appropriate for the destination) you will learn how things should *sound* and you can mostly forget about meters.

3 to 6dB of headroom is appropriate for unmastered mixed material - never deliver maxed out or peak-limited mixes to a mastering engineer.