[MAzevedo]

I do this all the time, especially when someone has gone crazy with buss compression on their mix and now the acoustic breakdown is louder than the chorus. I usually cut the pieces into regions and change the segment gain rather than mess with master fader automation, but it's the same end result.

As to how much, well, as much as it takes. I generally will make these changes in the source files before I add my compression. Usually all it takes is 1-3dB changes but there have been times I have moved things much more than that. Like most things in mastering, if it sounds right it is right.