Need information about 2x, 3x,.. harmonic generation analysis using Fourier transform

I can perform Fast Fourier Transform on sine waveform and obtain some spectrum with Nx harmonics, using pinch-off or saturation of sine. Is there any way to quantitatively predict harmonic level of certain device (FET, diode) at some harmonic in dBms? For example, in paper "Stable X-Band Local Oscillator, Eric A.Craig", there is x5 multiplier using FET transistor. Fig2. shows classic picture from books about conduction angle vs harmonic level. But more interesting is fig.5 "Multiplier spectral responce". How to obtain such spectrum for Pin=...dBm?