Left: contour plot of the ratio (ΔR_i,M/ ΔR_i,Z), i.e. the ratio of the variation of the inner boundary R_i due to the change of the host star mass to the one due to metallicity change. Mass and metallicity are changed within their uncertainty ranges (see text for details). Right: as in the left panel, but for the outer radius R_o. We adopted ΔY/ ΔZ = 1.