Hi Jeff,
Your dialog levels should be around -12db to match up with broadcast standards, I would not change the master level from the default 0db. The best way that I have found to level the volume on a project is to individually adjust each sound clip by dragging the volume bar on the clip itself. You will notice that this is usually set to default 0db, therefore you can only decrease the volume by pulling the clip level down. If you need to increase some clip levels to match the ones before and after, normalize them and then adjust the levels down to match. You can also use envelopes to adjust the volume, but in my experience I have found the method above to be easier to manage, especially when you are likely to fine trim events again later on. You can also use compressors and limiters to even out the volume on your track, but I would always start with adjusting the clips themselves first.
As regards the master track, This is useful for adding Audio fades at certain points affecting all audio in the project using envelopes, for example between scenes where you may wish to lower music dialog and fx all together to transition to the next scene.
Hope this helps
Dave