  Any asymmetry in energy of the colliding beams will lead to a longitudinal
boost of the center-of-mass frame of colliding particles w.r.t. the laboratory
frame and consequently to the counting loss in luminometer due to the loss of
colinearity of Bhabha final states. At CEPC running at the Z0 pole, asymmetry
in energy of the colliding beams should be known as well as 12.5% of the
beam-spread, in order to control the uncertainty of Bhabha count at the level
of 0.01%. Here we discuss the method, initially proposed for FCCee, to
determine variation of the beam-spread from the measurement of the effective
center-of-mass energy in $e^+e^- \to \mu^+\mu^-$ collisions.
