Turning ADC 10 bit to voltage different ways, it's okay to use what fits your app.
mV = Vref * ADC / 1024
mV = Vref * ADC / 1023
mV = Vref * ( ADC / 1024 + 1/2 step ) ---- best guess
I see far more people use analog to check the rough position of a pot. They don't need to calculate volts but so many do!
If the analog read is to be processed with a fixed calculation or one from several then I would pre-calculate and table all the possibilities to look up. If the result is a float then think of the cycles saved. The result could as easily point to a function to run.