Results from testing the method described in Sect. 3.1.2 measuring the FW3414(Hβ). Top panel: four illustrative Hβ profiles named A, B, C, and D for FASTWIND models with T_eff = 24 000 K and R = 25 000, and different pairs of log g and v sin i. The horizontal pink lines indicate the position at three-quarters, one-half, and one-quarter of the line depth for example D. Middle and bottom panels: log(ℒ/ℒ_⊙) against FW3414(Hβ) and FWHM, both measured in a grid of synthetic Hβ profiles from FASTWIND with the same T_eff and R as in the top panel, and values of v sin i between 10 and 360 km s⁻¹. The four profiles in the top panel are indicated with open circles and letters. The error bars in the middle panel show the average shift in all measurements due to the effect of different resolutions, and the effect of increasing T_eff up to 30 000 K (approximate temperature for a B0 I–II star).