Before revision: For audio-band signals, this can even be done "by ear"; for example, a -40dB reduction in the 3rd harmonic corresponds to setting the duty factor to 1/3 with a precision of 1% and -60 dB reduction corresponds to a precision of 0.1%.
After revision: For audio-band signals, this can even be done "by ear"; for example, a -40 dB reduction in the 3rd harmonic corresponds to setting the duty factor to 1/3 with a precision of 1% and -60 dB reduction corresponds to a precision of 0.1%.