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Adding reservoirs as extensions to a multi-cavity runner-system to regulate cavity fill-rate within small, high-precision optical parts during filling and fill-to-pack switch-over (F/P) was studied with the aid of simulations. This work aimed for a constant melt-front velocity inside the cavities with varying geometries. Three methods of reservoir designs were considered: first, using engineering intuition, second and third, using mass balance and mass and momentum balance equations, respectively. Eight reservoirs were designed and compared to two no-reservoir cases. For each case, 27 runs covering three levels of fill-rate, F/P, and packing pressure were simulated, resulting in 270 simulation runs. The quality variables of flow and thermally induced retardation and the average and standard deviation of volumetric shrinkage were considered and for each parameter, the minimum, best cases, occurred with a reservoir case. Thus, this study offers a proof-of-concept design for using reservoirs to improve molding of small, high-precision optical parts.