[parkher]

Distance:

The shorter the distance the more strain to the eye muscles.
That's why I think less than 40 cm is bad.
Even when reading printed books.
It is possible to comfortably read books (including ebooks on a 6" device) from 40-50 cm distance, even when using rather small fonts: 20-21 points. The range of font sizes in the printed books I have (mostly American paperbacks) is 19-25 points although I've seen even larger sizes.
I would not go below 19 points, if you need smaller fonts, the screen is too small. Throw away that device.
(I'm not sure if the size in points is absolute enough, I compared Amasis font on T1 with various printed books. Also DejaVu on PB902 looks the same size)

The minimal distance imposed by the color sub-pixels on JBC is a different matter.

Lighting:

I suggest having excessive lighting with the possibility to reduce it.
I have 5 x 4 W 400 lm LED lights (so, 20W 2000 lm) from a rather close range (about 1 meter).
It is equivalent to (almost) a 150W incandescent @ 220V.

So, when you increase the strength of the lighting the screen looks whiter and whiter.
But at some point it begins to hurt the eyes like a LCD - it feels like it is emitting the light itself. So then you reduce the light (or direct it partially away) until it is perfectly comfortable to read.
Furthermore, it is a good thing to choose the color (temperature) of the light, if possible, in such a way that the screen looks really white or just a bit yellowish like a real paper, to get rid of the blueish-greenish tint.
Then you have the whitest screen possible.
I think at home you should always have dedicated lighting for reading, you always should have perfect lighting conditions. Even if you read only printed books.

I realize that if JBC is used for textbooks at schools, then you don't have control over the lighting conditions.

So there are two questions about JBC:
- is it good enough under perfect lighting conditions?
- is it good enough to be used in schools.