I think c) . Just calculate your capital with the square root rule from the manual, then use the backtest for setting the Margin slider so that it matches the capital. From your posts I see that you have understood how to do this. The other formula was just the rule of three for quickly adjusting the Margin slider to a new investment. It was not for updating to a new version. If it causes trouble and confusion then just forget it and set Margin by backtest.