Humans can perceive external forces applied on a grasping tool based on skin pressure distribution at multiple contact areas during grasp. We have developed the TAKO-Pen, a pen-type pseudo-haptic interface using suction pressure stimulation on the each skin. In this paper, we investigate an improved method of skin stimulation based on a combination of psychophysical evaluations and mechanical simulation. We focus on a simplification method of the complex strain energy density (SED) distribution at the contact areas with a four quadrant values (SED index). The relationship between suction pressure and SED index was achieved by connecting the evaluations and the simulation. Moreover, we confirmed that an appropriate SED index could represent the magnitudes of forces in multiple directions with a linear function. Experimental results also showed that the proposed method could represent arbitrary directions between a pair of the orthogonal axes.