you might find the info on my site useful
Precise voltage measurement with an Arduino microcontrollerfloat voltage = sensorValue * (5.0 / 1023.0);
Actually, it is not useful and continues to muddy the water. This material has been debated endlessly in this forum, and neither value in the code line above is correct. Not the 5.0, and not the 1023.
For accurate results the "5.0" must be replaced by a calibrated or measured value, and the correct divisor is 1024.
A sensible, thorough, thoughtful and genuinely useful discussion of the standard Arduino ADC, including a discussion of how the results are to be interpreted (scroll far down the page for interpretation) can be found at Gammon Forum : Electronics : Microprocessors : ADC conversion on the Arduino (analogRead)