I believe there are simply too many variables, including sensor sensitivity, for this to be really worth doing. Better would be to simply try the lenses out and compare, but it can feel like splitting hairs.
"Some lenses, mainly those used in motion cinematography and not still photography, have apertures rated in T-stops. T-stops indicate the actual or absolute amount of light being passed through (transmitted) the aperture diaphragm. They differ therefore from f-stops, which indicate the aperture size relative to the focal length. (in a sense, T-stops are absolute and f-stops are relative)."
I.e., as you change focal length f-stops become relatively smaller as you go wide and larger as you go long. Now two 28mm lenses may transmit different amounts of light at f4 due to a differing number of elements, but additionally, you might have a problem if you go wider, as f4 is actually smaller. Correct me if I am wrong, but it seems that around 50mm is a "sweet spot" where you have the ability to made the front element quite large due to the normal field of view (i.e, light does not need to bend as much as a wide-angle). Additionally the design allows for super-wide apertures. I've heard that 85mm 1.4's are essentially light-vacuums, but good luck using them indoors comfortably!
Compare the front element of your 50mm and your 21mm and you might see what I mean.
In any case, I would suggest that it is the focal length's fault here, and maybe not transmittance. I would go for a narrower focal length, maybe 35 would be a good compromise for indoor work. I know there is the FA 35 2.0, and the new DAL 35 2.4. If you can, try them out before you commit.
And please keep the 21, it's supposed to be a wonderful lens, albeit not exactly a low-light wonder
.
Hope this all is accurate!