The correct input voltage is V = x / 255 * 5.
Your error is very common, and I found it in books and tutorials.
With 8 bit you can code decimal numbers from 0 (=0b00000000) to
255 (=0b11111111).
This equation is even better: V = x * 5 / 255.
Your equation may be useful to avoid floating-point calculations (add
x five times and left shift all the bits by 8 positions) but I prefer to
avoid other errors in the measurement, besides noise,
tolerances and so on.
Supposing that the input voltage is 4.2 V, just consider the integer
number 42. You must store each digit in an unsigned integer variable.
Each display must be turned on at one time, displaying each digit.
Your eyes will see all the digits displayed, if that operation is done
with a certain frequency. The displays are "multiplexed". If you can't
imagine this tecnhique working, look at this nice picture:
www.eeng.biz/multiplex.htm
In order to display digits on the display you must know the pin lay-out
and what type they are (common cathode or anode). Each pin turn on/off one
LED of the display. If you want to show number '8' all the LEDs, except the
dot point, must be turned on, and so on...