How to represent ratio between harmonics in audio spectrum
I think what you are referring to is called the Total Harmonic Distortion (THD). THD is the level of the fundamental divided by the level of the harmonics.  You can express this as either as a ratio (X/Y), a percentage (X/Y * 100) or in dB (20*log10(X/Y)). A percentage is probably what most people are familiar with.