As stated:
"The A/D read 255 full scale. So if your divider divides x volts to full scale the the A/D will read

255/x*Vin

Then you take that value and multiply/divide or whatever to make it input volts."



detailling it:

Let us assume that the A/D gives full scale for 3 Volts. I do not have the time to hunt the value from the datasheet.
Now, let us assume that your MAX input voltage is 15Volts, thus your divider could become 4k and 1k.

So when you apply x (15V) the A/D get full blast and will read 255.

Then if you apply Vin = 7.5 Volts the A/D will get half blast and read 127 or 128.

Try a few examples with the formula above, You'll see.

Erik