Actually I realise in reviewing this to explain it that I made an error earlier!
First of all visualise your data, It is a straight line as we only have two points.
ambient light = 280, brightness = 255
ambient light = 350, brightness = 51 (ie 20% of 255)
Plot it on a piece of paper.
Every straight line can be defined by the formula
y = (slope * x) + constant
so in this case
brightness = (slope * lux) + constant
Now the slope of the line is:
(51-255)/(350-280)
= -204/70
= -2.91 (I got -3.36 last time, don’t worry how, I just did, sorry bout dat)
To work out constant, take one of those points we know about and work it out. We’ll use the left hand point, lux = 280, brightness = 255
We know
brightness = (slope * lux) + constant
255 = (-2.91 * 280) + constant
255 = -814.8 + constant
so constant = 1069.8 (once again different to last time because of the wrong answer for slope)
Try it out for the other point, just to check:
lux = 350, brightness = 51
brightness = (-2.91 * lux) + 1069.8
51 should equal (-2.91*550) + 1069.8) (in fact it is .3 out because of rounding errors, but that is acceptable.
So in short
brightness = (-2.91 * lux) + 1069.8