Hello,

If it's only a sinewave you're intersted in, simply measure the average voltage and multiply it with the appropriate formafactor, simply by calibrating the input voltage.
If you want to have a true RMS reading, it is not enough to measure only a quarter of the wafeform. But measuring the complete wafeform at a reasonable high speed is an option. Always measure long enough, and preferable a multiple periodes of the 50 or 60 Hz mains frequency to reject common mode error. Square every sample, and add them together. At the end of the measuring interval divide the squared sums by the number of samples taken. Then take the square root to get the final result.
An other option is to use a hardware true RMS converter, like one from Maxim (www.maxim-ic.com). Free samples are available there!

Happy programming