Standard deviation in LTSpice?

I am wondering if anyone found a way of extracting the standard deviation of a plot within LTSpice i.e. not having to resort to Excel or Minitab for the effect?

I've kludged a method whereby...

.MEAS MyMean AVG MyPlotVar

.MEAS Sigma RMS (MyPlotVar - MyMean)

unfortunately the resulting sigmas are a bit off by about 10% to 15%... no idea where the error comes from as I don't have visibility of how LTSpice handles data which is a bit frustrating...

Thanks for your comments

Nando