What is the procedure for reflectance calculation from Sentinel 2 1C products downloaded from ESA Scientific Hub?
Sentinel 2 L1C product are top of atmosphere reflectances. It mean that ESA has already converted the raw DN (Digit Numbers) from the satellite MSI sensors into meaningfull physical values. This is a calibration step that includes a conversion from DN to luminance (based on sensor calibration model) and a conversion from luminance to reflectance (using solar irradiance in each wavelength at the time of acquisition). As mentioned by @Luke, the resulting reflectances are multiplied by 10000 (this is a technical reason for better storage)
L2A images are now also available. Those are Top of canopy (also called bottom of atmosphere) radiances. The SEN2COR algorithm is used for this purpose (available from SEN2toolbox if you want to do it yourself.)