GoForSmoke:
Divide the output by 1023 and it gets mapped to whatever you scale it to, because sometimes it is VREF and what if it's 100 mV more? Still 1023.
Scaling? No. What you are doing is introducing a gain error. The simple fact is that the slope of the line for an AVR analog-to-digital converter is always 1 / 1024. By using 1 / 1023 you are first converting using the "front" of the steps gradually moving to the "back" of the steps. That is a mistake. The end result is an 0.10 % gain error.