Mechanical compliance is a key design parameter for dynamic contact-rich manipulation, affecting task success and safety robustness over contact geometry variation. Design of soft robotic structures, such as compliant fingers, requires choosing design parameters which affect geometry and stiffness, and therefore manipulation performance and robustness. Today, these parameters are chosen through either hardware iteration, which takes significant development time, or simplified models (e.g. planar), which can't address complex manipulation task objectives. Improvements in dynamic simulation, especially with contact and friction modeling, present a potential design tool for mechanical compliance. We propose a simulation-based design tool for compliant mechanisms which allows design with respect to task-level objectives, such as success rate. This is applied to optimize design parameters of a structured compliant finger to reduce failure cases inside a tolerance window in insertion tasks. The improvement in robustness is then validated on a real robot using tasks from the benchmark NIST task board. The finger stiffness affects the tolerance window: optimized parameters can increase tolerable ranges by a factor of 2.29, with workpiece variation up to 8.6 mm being compensated. However, the trends remain task-specific. In some tasks, the highest stiffness yields the widest tolerable range, whereas in others the opposite is observed, motivating need for design tools which can consider application-specific geometry and dynamics.