Three-panel figure showing our best estimates of the overall uncertainties in T_eff, log g, and log n(He)/n(H) (from left to right), displayed as scattered markers. The T_eff uncertainties are plotted as fractional values (ΔT_eff/T_eff, i.e. 1–5%). Helium-rich and helium-poor stars are separated and indicated as orange squares and dark blue circles, respectively. The solid orange and dark blue lines are polynomial fits (Equation (1)) to these scattered markers, and the coefficients of these are given in Table A.2. The dashed polynomials represent the identified uncertainty contributions from the intrinsic scatter measured in our INT data and from the metallicity abundance uncertainty, as described in Sect. 4.2. The formal statistical errors are given by the error bars of the scattered markers and the instrumental offset between INT and ALFOSC is given by a black-dashed line. All four identified uncertainties are added together in quadrature to arrive at the uncertainties used for the analysis in this work.