Leakiness (ϕ), the proportion of carbon fixed by phosphoenolpyruvate carboxylation that leaks out of the bundle-sheath cells, determines C₄ photosynthetic efficiency. Large increases in ϕ have been described at low irradiance. The underlying mechanisms for this increase remain uncertain, but changes in photorespiration or the energy partitioning between the C₄ and C₃ cycles have been suggested. Additionally, values of ϕ at low light could be magnified from assumptions made when comparing measured photosynthetic discrimination against ¹³C (Δ) with the theoretical formulation for Δ. For example, several simplifications are often made when modelling Δ to predict ϕ including: (i) negligible fractionation during photorespiration and dark respiration; (ii) infinite mesophyll conductance; and (iii) CO₂ inside bundle-sheath cells (C_s) is much larger than values in mesophyll cells (C_m). Theoretical models for C₄ photosynthesis and C₄ Δ were combined to evaluate how these simplifications affect calculations of Δ and ϕ at different light intensities. It was demonstrated that the effects of photorespiratory fractionations and mesophyll conductance were negligible at low light. Respiratory fractionation was relevant only when the magnitude of the fractionation factor was artificially increased during measurements. The largest error in estimating ϕ occurred when assuming C_s was much larger than C_m at low light levels, when bundle-sheath conductance was large (g_s), or at low O₂ concentrations. Under these conditions, the simplified equation for Δ overestimated ϕ, and compromised comparisons between species with different g_s, and comparisons across O₂ concentrations.