there's an error in the docs at https://sentinel.esa.int/web/sentinel/sentinel-1-sar-wiki/-/wiki/Sentinel%20One/Application+of+Radiometric+Calibration+LUT#section-Application+of+Radiometric+Calibration+LUT-ApplicationOfDeNoiseLUT. Corrected by dividing by the square of sigmaNought LUT. Not sure if the noise correction is correct but will ask ESA. It is anyway very small.