Using the Mapping Method different designs of SMX motionless mixers are analyzed and optimized. The three design parameters that constitute a specific SMX design are: The number of cross-bars over the width of channel, N-x, the number of parallel cross-bars per element, N-p, and the angle between opposite cross-bars theta. Optimizing N-x, somewhat surprisingly reveals that in the standard design with N-p = 3, N-x = 6 is the optimum using both energy efficiency as well as compactness as criteria. Increasing N-x results in under-stretching and decreasing N-x leads to over-stretching of the interface. Increasing Np makes interfacial stretching more effective by co-operating vortices. Comparing realized to optimal stretching, we find the optimum series for all possible SMX(n) designs to obey the universal design rule N-p = (2/3) Nx-1, for N-x = 3, 6, 9, 12,....