Colloidal quantum dots are promising next-generation phosphors to enhance the color rendition of light-emitting diodes (LEDs) while minimizing the brightness droop. In order to exploit the beneficial tunability of quantum dots for highly efficient devices, optimization and determination of the performance limit are of crucial importance. In this work, a facile preparation process of red-emission quantum dot films and simulation algorithm for fitting this film with two commercial LED flat lamps to the optimized performance are developed. Based on the algorithm, one lamp improves from cold-white light (8669 K) with poor color rendition (Ra = 72) and luminous efficacy (85 lm/W) to warm-white light (2867 K) with Ra = 90.8 and R9 = 74.9, and the other reaches Ra = 93 ∼ 95. Impressively, the brightness droop is only about 15 ∼ 20% and the luminous efficacy of 68 lm/W is achieved. Furthermore, our device shows reliability over 1000 hours with only PET (polyethylene-terephthalate) films as the barrier, indicating that this auxiliary red-emission film can be easily applied to improve the color rendition of most commercial LED flat lamps.