I do not know where you get the SDR data. if it is in HDF5 format. It is straightforward. First, you have to read "/All_Data/VIIRS-M2-SDR_All/Reflectance" (as ref) , then read "/All_Data/VIIRS-M2-SDR_All/ReflectanceFactors" (as scale). then applying the scaling factor to the reflectance is the reflectance you wanted, as following