Dye-doped chiral-nematic liquid crystal lasers have great potential as small size, low-cost, widely tunable lasers. We present a numerical model for stimulated emission and lasing in liquid crystal films based on thin film optics. The gain threshold is modelled and the results are confirmed experimentally. The effect of the orientation of dye molecules and the matching of the photonic bandgap to the dye spectrum on the threshold for lasing is discussed.