TolpuddleSartre:
Or, put more succinctly, you could RTFM, and see what the device's own manufacturer recommends..
Except that Atmel was not brave enough to explain (in the Atmega 328 datasheet at least) how to convert an ADC reading back to the voltage from which the reading was obtained.
I don't think any of the subsequent comments have improved on @CodingBadly's Reply #75
...R