The standard deviation sum-of-fractions for building surfaces is required for computing the number of samples and is calculated as follows:
\begin{equation} S_{\text{sof}} = \sqrt{\Big(\frac{S_{alpha}}{DCGL_{alpha}}\Big)^2 + \Big(\frac{S_{beta}}{DCGL_{beta}}\Big)^2 + \Big(\frac{S_{gamma}}{DCGL_{gamma}}\Big)^2} \end{equation}
Where \(S_{alpha}\) is the total standard deviation for the alpha activity and \(DCGL_{alpha}\) is the gross DCGL for the alpha activity, etc.
You can also choose to enter the estimated standard deviation manually by setting the Manual / Auto Selector.