bandgap from UV/Vis spectra

 we're trying to 'guess' a bandgap from our calculated
 UV/Vis spectra, but instead of using the first peak
 maximum (lowest energy) I've heard that experimentalists
 working on polymers take the crossing of the tangient
 at the point of inflection at the low energy side.
 Now my question is if someone knows why they proceed
 like this and more importantly where I can find a
 reference about this ?
 Thanks in advance .. I'll summarize if someone would
 be interested in this ..
 	Sergiusz Kwasniewski
 	Universitaire Campus Gebouw D
 	3590 Diepenbeek
 	tel(direct): 032 (0)11/268315
 	fax	   : 032 (0)11/268301
 	email      : sergiusz.kwasniewski.,at,