Accuracy of RMS of sine measured from N samples of ADC conversion
In order to fully reconstruct the sine wave and avoid aliasing problems, your sampling rate should be at leas the Nyquist rate which is twice as the sine wave's frequency. Assuming this condition holds, you can fully reconstruct the sine wave(up to ADC's resolution) and calculate the RMS. The ADC resolution along with your scaling factor will determine the accuracy of the measurement. If we assume that the resolution is X mV, the RMS error will be X/sqrt(2) mv.