Robin2:
Using 1024 has the advantage that the maths can be much faster (but not using map() ).
When you make the mistake to convert ADC result to float you are wasting so much time. The improvement gained by using /1024 instead of /1023 is small if any.
If you want to make the code fast you should use raw ADC result and avoid the division at all. If you want easily readable result you probably do not care about speed (presenting the result to the user will be more likely your bottleneck).