The slides Systematics.pdf show the systematic uncertainties for the preliminary gains. 
The E/p values used to calculate the systematic uncertainties are calculated in the same manner as the preliminary gains (JP and BHT2 E/p are determined separately, then averaged).
Current estimate for the total systematic uncertainty is 2.5%
The systematics are not complete though. There needs to be at least one more additional systematic uncertainty associated with the procedure used to extrapolate the JP-BHT2 difference into the outer rings (for which the actual BHT2 sample is not usable). This is likely to be a significant uncertainty which will increase the total 

Avenues to improve the systematics:
Determining the reason for the momentum dependence of the JP sample. 
Determining the reason for the momentum dependence of low momentum tracks for the outer rings.
Determining the reason for the significant difference between the JP and HT triggers.