What's the right scale to represent a frequency spectrum?

To represent a 16 bits temporal spectrum, it's "easy" : my scale is going from -32000 to +32000. But What's the scale for a frequency spectrum ?

I made that try : I calculated a pure single frequency sine signal and calculated it's energy for a period. So, I supposed that any signal generated on such a period - without saturation, of course - could have more energy. So, I considered the ratio

20.log10 (signal_energy/maximum_theorical_energy)

as my 0 dbFS.

Then, I made few experimentations, generating signals for different kind of frequencies and, when I had a single sine, each time, I reached the 0dbFS.

But, of course, a music have thousands of frequencies. So, when I represent a spectrum of a "music sound" signal, I rarely reach more than -30 db and my signal is more or less around -60 db for each frequency. It seems very low.

So, my question is : is it correct to represent a signal with such a measurement ? If not, how do represent the "values" for each frequencies on the graph ? With which scale ?