To develop a thin, high image quality, and low power consumption eco-display, a novel backlight determination method called "Limited Backlight Signal Ratio" (LBSR) is proposed to applied in a side-lit 180Hz Stencil-FSC LCD. The point spread function of a side-lit backlight division is optimized to maintain a 97% higher backlight luminance uniformity and good image fidelity. Simulation results indicate the optimal backlight division is 2x16 with color breakup suppression reach asymptotic end values. The experimental photos demonstrate the side-lit LBSR 180Hz Stencil-FSC method can suppress color breakup effectively and can increase image fidelity greatly. As a result, side-lit LBSR 180Hz Stencil-FSC method is promising for future side-lit eco-display applications.