Generally it is the minimum diameter of the GrowCut algorithm that has to be adjusted for local threshold as the effect is more so a combo effect that is “Threshold+Grow From Seeds”.
@Thomas_K, @lassoan, @pieper and myself diiscussed this same topic about the confusion of the effect at the following hangout (linked below) with Thomas proposing a “Fast local threshold” which always picks the selected voxels such as a “Threshold+Islands” type of combo effect rather than being only based on the Grow From Seeds approach. That way what you click is what you get. From that discussion it seemed that @lassoan recognized the confusion in the current effect, but there was no clear consensus about the integration of such a “Threshold+Islands” type approach.
The regular “Threshold” effect supports local histogram selection (drawn below), but then you have to use another effect to ultimately pick which selected voxels to keep from the full selection.
