Our Standard Deviation formula does use N-1. You can see more details by looking in your PingPlotter Pro\scripts directory and look at the "Std Deviation.ppx" file, which is the script used to calculate standard deviation (and display it, in case you want to change the red threshold point). You're free to tweak this formula yourself, although I'd recommend copying it so the next upgrade to PingPlotter Pro won't overwrite your changes.