log(g f)_grid values and distributions for the λ5682.2 Ni I line. Top left panel: individual log(g f)_grid values per star, including 68.3% confidence-limit error bars, plotted against benchmark T_eff. The solid black line denotes the weighted and the hatched region denotes the standard deviation estimation, discussed in Sect. 4.3.8. Remaining panels: distributions of the synthesised grid of λ and log(g f) values for the seven benchmark stars. Calculated grid points are denoted by black dots, the individual log(g f)_grid values are denoted by black crosses, and the 68.3% confidence limits are shown with black contours. Each plot has a normalised colour-scale, where yellow represents the minimum, and dark blue represents the maximum.