Fine-tuning of stellar parameters (here, T_eff), for the case of HD 36512 (O9.7V). The stellar parameters initially estimated from the H/He lines (green) still need some additional fine tuning, since some of the carbon lines are much more sensitive to small changes than H/He. The color coding for T_eff is as follows. Green: 33 kK (see Holgado et al. 2018), blue: 33.6 kK, turquoise: 33.8 kK, and red: 34 kK (the latter displaying too much C IV).