The presentation 190208_AMIncJet_Pol.pdf summarizes the calculation of the luminosity-weighted average polarization for the pp_long_2015 dataset together with the estimation of the uncertainty.  

The values taken as fill-to-fill scale uncertainties, scale uncertainties, and uncertainties due to the profile correction are from the RHIC polarimetry group report: https://wiki.bnl.gov/rhicspin/upload/e/e4/Pol_resultsMay2016.pdf
The values of the polarization at the beginning of the fill, and the polarization decay (polarization coherence constants) are taken from: https://wiki.bnl.gov/rhicspin/Run_15_polarization