code: public static double signedDistanceToPlane(Vector3d planeNormal, Point3d pointInPlane, Point3d point) {
          if (planeNormal == null) return Double.NaN;
  
          Vector3d pointPointDiff = new Vector3d();
          pointPointDiff.sub(point, pointInPlane);
          return planeNormal.dot(pointPointDiff);
      }
docs: Given a normalized normal for a plane, any point in that plane, and
  a point, will return the distance between the plane and that point.
  
  @param planeNormal the normalized plane normal
  @param pointInPlane an arbitrary point in that plane
  @param point the point to measure
  @return the signed distance to the plane