Do you really want to calculate the RMS? Assuming your mains is sinusoidal you can just scale your readings to show the RMS value (peak/0.71) which is what the cheaper multimeters do. If you wanted to measure the rms votage from the output of a phase controlled light dimmer - then you would need to calc rms. If all you want to do is measure the minimum and maximum voltages on the mains then just rectify the ac and scale for the equivalent rms reading as would a normal multimeter.