FWHM (Top), contrast (Middle), and area (Bottom) of a Gaussian fit to the residual profiles. The Fe i 610.8 nm, 616.5 nm, and 617.3 nm simulated data are plotted as grey, brown, and pink lines, respectively; line style indicates the average magnetic field strength, with solid, dotted, and dashed lines representing 20, 100, and 500 G simulations, respectively. HD 189733 data are colour-coded by phase, with error bars colour-coded by observation night. Black horizontal bars represent the estimated error on the observed ⟨μ⟩ at different locations. Inset displays the reduced χ².