For myself, most of the time my monitors are set to a maximum of around 30 on a scale of 100, and I don't get any eyestrain. Dark mode with a properly calibrated monitor, in contrast, does give me eye strain - the light letters on the dark background don't play well with my Keratoconus.
Right now, with HN defaults, at around 7pm, the brightness is 15, and contrast is 30.