Assume n is the normal vector returned by pcfitplane.
If you want n to point generally up from the ground, then you must know z, the vector that points vertically up. So you do
if dot(n,z)<0
n=-n;
end
If you want n to point away from the center of a cube, then you must know cc, a vector for the approximate center of the cube, and vector cp, which is the centroid of the ponts that were used to fit the plane. (The centroid of the points used to fit the plane will always be in the fitted plane.) Then do
if dot(n,cp-cc)<0
n=-n;
end
Good luck.
