If you're using the Create Custom Geographic Transformation tool or the New Transformation dialog in ArcMap, you only need to define it in one direction. In fact, it's very slightly more accurate to define the transformation as-given, because then the software can invert the transformation's rotation matrix (more necessary when the rotation values are relatively large).
If, for some reason, you do need the parameters for the opposite direction, simply change the signs so:
dx(m) = -89.5
dy(m) = -93.8
dz(m) = -123.1
rot(x) = 0
rot(y) = 0
Rot(z) = +0.156
Scale = +1.200
Melita