Required accuracy (color coding) of perturbed Z_p, p = 1,...,5, such that for at least 2/3 of the best-fit pairs (β_f, Δ_f) the associated C_f are within 10% (top) or 5% (bottom) of the C associated with the initial, unperturbed (β, Δ) pair. As can be seen, the required accuracy depends crucially on the location within the β-Δ plane. Gray lines indicate the range within which 2/3 of the actual β_f (vertical direction) and Δ_f (horizontal direction) lie. Black lines indicate constant C = 1 (lower line) and C = 2 (upper line). For details see text.