plot the values of (αhυ)^2 vs hυ for direct band gap & for indirect band gap (αhυ)^1/2 vs hυ. And the choose straight line vertically downward to X-axis that value is your band gap. if you need any standers of excel sheets you can contact and I will send to you, you just put the values of your absorbance or transmittance with the wavelength then directly you get the results just