Previous work found that using 8 screened cutoffs of approximately 50-66% of the planewave cutoff yielded high-quality RPA correlation energy extrapolations. However, these results were calculated using PAW pseudopotentials. To our knowledge, similar calculations using the SG15 pseudopotential set that might require different planewave cutoffs have not been performed. The polarizability was constructed using different numbers of unoccupied bands. The number of unoccupied bands could held constant and defined by the planewave cutoff, or defined by the screened cutoff and varied with each point in the extrapolation. A large range of screened cutoffs were tested to quantify different extrapolation and polarizability convergence approaches. We used the atomization energies of 12 small molecules as a benchmark comparison.