Uncertainty in the vertex deviation σ_lv as a function of sample size, N. Stars are sampled from fields in Fig. 8. We show the S-NSB (red) and W-NSB (green) cases, both with α = −60°. The uncertainty decreases steeply up to N = 500, then more gradually thereafter. The W-NSB case yields systematically higher uncertainties (by ~8°) compared to S-NSB at all sample sizes due to its more isotropic velocity distribution (lower β).