Being able to perform a piece of music competently with expression is a function of the music's technical demands and the player's skill. Each musician is faced with unique challenges that change as repertoire goals and proficiency levels change.
I often find it useful to determine my "natural tempo" for a piece of music after the technical issues have been resolved. I will basically ask myself "at what tempo can I play this piece well without feeling much stress?"
First, I play the piece without the metronome at a relaxed pace until I can execute it cleanly. This pace may be dead slow or not, depending on the music's complexity. I then reference the metronome to find my natural tempo that I can use to practise my piece. I find that jotting down the tempo with the date is helpful to note progress later on.
My natural tempo shows me how far I am from my goal in real terms. I can explore expressive elements of the piece because technique is less of a concern to me. Natural tempo can vary somewhat each day, but over time, it tends to increase organically to a certain point. At that point, I have to start pushing my tempo boundaries again so I can improve if I haven't already met my goal.
I realize that many musicians (myself included) do not have the luxury of time to learn music using the natural tempo method. However, I find this method useful when learning to play music that is of innate interest to me.
