It is well known that for a large reactor a diffusion calculation of the system eigenvalue (criticality) is weakly dependent on the linear extrapolation distance γ. We characterize this weak dependence by a smallness parameter ϵ, and show that the complete neglect of γ leads to an error in the computed eigenvalue of the order of ϵ, whereas the use of an extrapolated endpoint introduces an error of the order of ϵ2. An explicit formula, which preserves the ϵ2 error characteristics, is derived which gives an energy independent extrapolated endpoint in terms of the energy-dependent linear extrapolation distance.