Macs have long reserved ~50w of power for the display, whether or not all of it is used is a different question. Giving them the benefit of the doubt (and accounting for screens that can max out at 1500 nits), I think an upper bound of 40w is pretty close to the actual figure. OLED will always pull less power since it doesn't need a backlight, in contrast to the hundreds of backlights on newer Macs.