I have measured the screen brightness for different UIScreen.main.brightness settings with as many of the other Display & Brightness settings disabled.
With Brightness = 1.0, my iPhone 12 mini display white is 250 nits.
For Brightness down to about 0.5, the brightness fits 250*pow(UIScreen.main.brightness, 2.2) reasonably well. The 2.2 gamma is approximately the sRGB toe curve. Below that, the brightness is higher than this formula. I cannot fit this using any offset and gamma in the sRGB formula. This is probably a better thing for the user, as otherwise the bottom en of the Brightness control would be very dark, and they might not be able to see to turn it up again.
The 250 nits value is a nice round number. Do all devices of the same model fit the same number? Is there some way to get this value for recent models?
Is there a formula for the brightness function? Is it the same for all recent models? The 2.2 gamma function seems to work over the brightness range I expect people to work with. If Brightness is set to 0.75, for example, I can come up with a reasonable figure for the RGB=1 white brightness. But can we improve on that?