Future e+e- colliders require very accurate knowledge of beam energy, one part in 10^4 to 1 part in 10^5. One possible approach is laser backscattering. The challenge is to establish an absolute calibration of the resulting photon’s energy. Since there are no sufficiently accurate calibration sources at the relevant energy, we must extrapolate / interpolate. We want to quantify some of the fundamental limitations on this procedure.