Reflectance can be calculated by using the solar irradiance (also referred to as F0 / 'f-naught').  The solar irradiance for each band is in the source Level-2 (unmapped) data nLw attributes:

long_name: Normalized Water-leaving Radiance at 486.0 nm
scale_factor: 0.001
add_offset: 0.0
units: mW cm^-2 um^-1 sr^-1
coordinates: time longitude latitude
grid_mapping: coord_ref
standard_name: normalized_surface_upwelling_radiance_in_air_emerging_from_sea_water
_FillValue: -32768
reference: Gordon & Wang (1994), Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: A preliminary algorithm, Appl. Opt., 33, 443-452
solar_irradiance: 198.8655
solar_irradiance_units: mW cm^-2 um^-1

The relationship between Rrs and nLw is:

Rrs_nnn = nLw_nnn / F0_nnn* where nnn is the wavelength/band and F0 is the solar_irradiance attribute value for each band.

*F0_nnn in our use already includes pi

Note that the solar_irradiance is not in the 4km data attributes.  We'll be sure to correct that in the next reprocessing, but the SCINIR L2 contains the solar irradiance for each band and these do not change over the science quality processing (2012 to present) or from Level-2 to Level-3 processing: 

                nLw_410:solar_irradiance = 170.8027f ;
                nLw_443:solar_irradiance = 190.2609f ;
                nLw_486:solar_irradiance = 198.8655f ;
                nLw_551:solar_irradiance = 184.2347f ;
                nLw_671:solar_irradiance = 150.4539f ;
                solar_irradiance_units = "mW cm^-2 um^-1" ;

So you are able to use  the global Science Quality S-NPP VIIRS nLw products (Level-2,-3,-4) and calculate the Rrs.