Take a photo of the spectrum.
Insert the photo in the computer and open it with an image processing software (Gimp, Photoshop). Desaturate it by luminosity and convert it to *.fit file format.
Open the fit file with the FV software (a NASA's HEASARC shoftware) and take the spectral profile.
In the next picture, the spectral profiles of KMnO4 solutions are shown.
Having the spectral profile, we can calculate the absorption and carry out the calculations we want.
In the next picture, a standard calibration curve is shown.