Hi

it's quite simple

I can't copy past the equation but you should have it

it's in fact the multiplication of the "spectral irradiance of terrestrial sunlight" x by "the erythemal action spectrum" divided by "spectral irradiance of terrestrial sunlight" x by "the erythemal action spectrum" x by the "transmittance", for each wavelength between 290 and 400

spectral irradiance of terrestrial sunlight: is the irradiance àf the standard sun

The standard erythema action spectrum provides an internationally accepted representation of the erythema-inducing effectiveness of wavelengths in the UV part of the spectrum.

T is the transmittance

So if you apply the right procedure (please see these publicationsArticle Validation of an In Vitro Sun Protection Factor (SPF) method...

and https://doi.org/10.1111/ics.12625) you just have to enter the transmittance in the equation to have the SPF

Hope this will help

Best regards