The Pulay stress shows only a weak dependency on volume and the ionic configuration. It is mainly determined by the composition. The simplest way to estimate the Pulay stress is to relax the structure with a large basis-set (1.3 default cutoff is usually sufficient, or PREC=High in VASP.4.4). Then re-run VASP for the final relaxed positions and cell parameters with the default cutoff or the desired cutoff. Look for the line 'external pressure' in the OUTCAR file:
external pressure = -100.29567 kBThe corresponding (negative) pressure gives a good estimation of the Pulay stress.