[JeffreyKohn] “side from listening by the ear, there must be some measured way of getting the best result. What is that way?”
The Mixer window reads out the total volume as you play. If you don’t see it go to View>Mixer.
If you have already mixed several sources, the easiest way to control overall volume over time is to apply a volume envelope on the master track (hit B to bring that up).
Gary