For varying Audio sources, I find that the Audio Mixer is great for making those adjustments, at the Track-level. I only do this after I have done all my cutting.
Personally, I feel the way you're doing it now is the best way to do it in PP.
It wont be a guessing game once you do it a few times and of course its made easier if you look at the meter.
eg meter will show you that you might need + 6 db to make it peak at say... -3db. Easy. Add +6DB.
Why don't you use some of the normalization features. You can right click and tell the audio files, select all of them at once, to normalize peaks to a certain level. Then all files are starting from roughly the same point. You can also use the volume handles on the clips to adjust their volume.