Differential elastic actuators connect a motor and a spring via differential gears to a shared output shaft, offering a more compact solution for creating mechanically robust systems than series elastic actuators, with superior torque transmission at high frequencies. The key to maximizing DEA performance lies in the careful selection of stiffness, inertia, and damping values to meet specific requirements for performance and durability. We introduce a DEA design guide that utilizes open-loop torque-bandwidth for performance evaluation and the magnitude of impact-induced gear torque for robustness evaluation. This approach enables determining DEA parameters using closed-form equations, eliminating the need for simulations or extensive expert knowledge. The effectiveness of our method is confirmed through experiments with a reconfigurable DEA prototype.