\sum_{{\mbox{\boldmath $x$}} \in C}
\vec{r} \cdot \vec{s}({\mbox{\boldmath $x$}}) \sin {\mbox{\boldmath $k$}} {\mbox{\boldmath $x$}} \right)^2 \right],
\label{eq:4}$$ which, for ${\mbox{\boldmath $k$}} = {\mbox{\boldmath $0$}}$, reduces to an improved estimator for the susceptibility $\bar{\chi}$ in the high-temperature phase, $$\tilde{\hat{G}}({\mbox{\boldmath $0$}}) = \tilde{\bar{\chi}} = \frac{3}{|C|}
\left( \sum_{{\mbox{\boldmath $x$}}
\in C} \vec{r} \cdot \vec{s}({\mbox{\boldmath $x$}}) \right)^2.
\label{eq:5}$$ Note that we follow in the definition of the susceptibility $\bar {\chi}$ in the high temperature section the standard convention of omitting the $\beta$-factor. From the theoretically expected small ${\mbox{\boldmath $k$}}$ behavior of the inverse Fourier transform, $$\hat{G}({\mbox{\boldmath $k$}})^{-1} = c \left[ \sum_{i=1}^3 2 (1 - \cos k_i) + (1/\xi)^2
\right]
\approx c \left[ {\mbox{\boldmath $k$}}^2 + (1/\xi)^2 \right], \label{eq:6}$$ where $c$ is a constant and $k_i = (2\pi/L)n_i$, $n_i = 1,\dots,L$, one may extract the correlation length $\xi$ by measuring $\hat{G}$ for a few long-wavelength modes and performing least-square fits to (\[eq:6\]). In our simulations we have measured $\hat{G}({\mbox{\boldmath $k$}})$ for ${\mbox{\boldmath $n$}} =$ (0,0,0), (1,0,0), (1,1,0), (1,1,1), (2,0,0), and (2,1,0) (see Sec. 4). It is well known that by means of the estimators (\[eq:3\])-(\[eq:5\]) a significant reduction of variance can only be expected outside the FSS region where the average cluster size is small compared 