A/D Arduino converter

Hello, I would like to ask you very kindly about 10bit arduino converter. If I am using 2V as reference voltage then to calculate its resolution I can apply the following formula Vref/(2^N-1) or Vref/(2^N) ??? which one is correct??? . Moreover, If the converter give 512, to calculate the corresponding voltage value I can use this formula 512*(Vref/1023) and if it is giving 256 then 256*(Vref/1023) and so on ???? in other words is it correct to use 1023 or 1024 for 10 bit converter??? Thank you in advance for your response.

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