In the article i could have seen both the formula.

alpha = 1/t * [log10 (1/T)] - 1

or 

alpha = 2.3026/t * (log10 [1/T] - 2

if i am using the second the my bandgap energy value is lowering than the first one. 

Ex. 4.79eV for first one (for Tauc plot)

      2.8eV for second one.

     ( am i correct or not? )

So, 1. which formula can i use?

       2. is there any possibility of changing the value.