Here is an interesting paper on band gap calculation of liquid water. Take a look.

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332004000100007

You mean how to estimate (not calculate) the value of band gap from the experimental Tauc plot? See Figure 6 in the attached paper.

http://www.hydrogen.energy.gov/pdfs/progress11/ii_j_3_billo_2011.pdf

I think in this question he means if there is some standard procedure, for instance, regarding concentration, to obtain the absorption spectra and then plot the Tauc graphs.  

I am telling this, because a problem that I commonly face while trying to estimate band gap in powders in liquid dispersion, is that the band gap estimate through Tauc plot varies a lot according to the dilution of the dispersion.

This is a good question, I am waiting the answers

regards

Maytham