Without measuring transmission, you can not assume transmission to be zero. Even for an absorbing material, intensity decreases exponentially with thickness. Unless you know the absorption coefficient, you can not assess the thickness for transmission to be zero. In conclusion, either you measure transmission or you calculate by knowing the absorption coefficient, then only you can say that transmission will be zero in 4 mm pellet.
I have a question. I have a graph between %reflectance and wavelength. How to calculate absorption coefficient and refractive index from the data? Dr A. Kumar