This work presents a mixed simulation experimental method for efficiency optimization of a modular multilevel switched capacitor DC-DC converter. The results show that the optimal frequency deviates from the circuit's self-resonant frequency, commonly proposed in literature. To combine a fast-transient analysis with system-scale simulations, a detailed transistor and switching capacitor modeling is performed with dynamic parameter readjustment and precise measurement of the parasitic inductances, which depend on the physical layout of the circuit. The results are experimentally validated on a GaN-based nX converter prototype, showing good correspondence.