Hi chee,
By its definition, the baseband telecommunication system doesn’t use modulation scheme (i.e., it transmits a base information signal) and its spectrum is different (in most cases its twice shorter) then the spectrum of modulated signal. Not sure what min and max values you exactly mean… In general, the full width of pulse spectrum can be determined at the level of -20dB (or 99% of its maximum power). This width can be used for spectral efficiency calculations.