We study scatter involved in finite-size scaling of the conductivity and re
sistivity tensors resulting, respectively, from uniform essential and natur
al boundary conditions applied to domains that are finite relative to the s
ize of a heterogeneity. For various types of planar microstructures generat
ed from Poisson processes (multiphase Voronoi mosaics, composites with circ
ular or needlelike inclusions, etc.) we report a universal property: the co
efficient of variation of the second invariant stays practically constant a
t about 0.55+/-0.1, irrespective of the domain size, the boundary condition
s applied to it, the contrast, and the volume fraction of either phase.