Hey Bwts.

When you subtract means you get the mean is E[X-Y] = E[X] - E[Y] with Var[(X_bar-Y_bar)] = Var[X_bar] + Var[Y_bar] so the standard deviation of the difference of the means is given by SQRT(Var[X_bar] + Var[Y_bar]). So you add the two standard deviations of the mean (estimated or given) and take the square root.