We present an approach to quantifying the off-axis stiffness properties of parallel compliant mechanisms used in the design of mobile millirobots. By transforming the stiffness of individual flexure elements and rigid links comprising a compliant mechanism into a global coordinate system, we enable the formulation of an equivalent mechanism stiffness. Using that stiffness in concert with an energy-based performance metric, we predict the performance of a compliant mechanism subjected to a prescribed set of forces in the global coordinate system. We analyze a flexure-based Sarrus linkage and use the performance metric to improve the design by adding topological redundancy. Finally, our approach is experimentally validated by constructing and testing SCM Sarrus linkages in a variety of geometries and topologies and demonstrating agreement between the model and our experiments.