If I understand you correctly, you want to first obtain n and k from the reflectance and then calculate transmittance by the Lambert-Beer law. There is no straightforward way to the solution, since you have only one value (the reflectance) to obtain n and k. Instead you have to perform either a Kramers-Kronig analysis or a dispersion analysis assuming suitable oscillator models to find out n and k. Note that also transmittance is a function of both n and k and Lambert-Beer is most of the time not a good choice to calculate T from k or vice versa...