[ProDigit]

I like 10. 10 is about the max I'd set it to in broad day light.
But if I'm reading in a pitch black bedroom, even setting 1 is just too bright for comfort, once the eyes get used to the darkness.
If I open the curtains, and some ray of a street light enters the bedroom, setting 1 is good enough for me.
In a dim candle light dinner - style restaurant, I can clearly see when the device is on setting 1, or off.
The screen becomes somewhat blue-er.

For me a setting like '0.75' would have been awesome.

Anyway, I generally don't read with the light, turn it to 1 all the time. Eventhough it looks very nice on 10, during the day, it's just wasting the battery, because the screen is perfectly readable in day light without the backlight.
In fact, with setting 1, I never have to worry about being able to see the text, as in the dark the backlight is enough, and in the light, the environment light is enough to read!

One thing I do know, is that the screen looks nicest at a bright setting. However at that setting, it's not healthy to read.

I want to repeat myself in my previous statement concerning LCD screens, and the same applies here.
Focus your eyes above the reader, look in the distance, and have the reader in the edge (corner) of your eye, in such a way that you still see the reader, but can't really read the text anymore, and your focus is not on the reader.
Now adjust the brightness to blend in with your environment (too bright, and the reader will appear as a white square in your vision, too dark, and it will appear as a grey or black square in your vision).
Once you have found the light intensity of the reader, to be the same output as the environment around you, add between '0' to '1' steps of light, but no more than 4 steps of light on the device.

When done, you have set up the reader to be of most comfort for you to read.
In darker environments the light should be lowered, in brighter environments, the light reflection of the environment should make the screen more bright automatically; though you can still compensate by adding a tad more light with the backlight as you desire, though generally anything above weak light inside a house has enough lumen, to ignore the backlight of the device...
(yeah, I'm gonna keep on calling it backlight, since a speaker is not a speaker but a magnetic membrane loudspeaker (or something), an LP is not always Long Play, a video is not always a VCR casette, and a projector is not always a picture projector, but sometimes also a movie or computer projector, and thus, a light shining on the background of a screen I find it to be a backlight).