Calculating Google Analytics event value variance & standard deviation

Here's I calculated the var & stddev of an event value (X) tracked by Google Analytics:

1. Add a second event with the value X^2
2. Calculate the variance using this formula: Var = E(X^2) - [E(X)]^2  (note that Google Analytics already calculates E(X^2) & E(X), which are, in this case the event's average).