Estimate of the optimal number of patches/voids to stack using the catalogue of Sutter et al. (2012). Starting from the largest void and adding one CMB patch at a time to the stack, we find at each step N the minimum of the aperture photometry profile and multiply this value by to find the highest signal-to-noise ratio, assuming that the noise scales roughly as 1/. The vertical axis has been normalized to the best signal-to-noise ratio, obtained for 231 voids.