  A constraint on the error of leptonic CP violation, which require the phase
$\delta_{CP}$ to be less than $\pi/4$ for it to be distinguishable on a $2\pi$
cycle, is presented. Under this constraint, the effects of neutrino detector 's
distance, beam energy, and energy resolution are discussed with reference to
the present values of these parameters in experiments. Although an optimized
detector performances can minimize the deviation to yield a larger
distinguishable range of the leptonic CP phase on a $2\pi$ cycle, it is not
possible to determine an arbitrary leptonic CP phase in the range of $2\pi$
with the statistics from a single detector because of the existence of two
singular points. An efficiency factor $\eta$ is defined to characterize the
distinguishable range of $\delta_{CP}$. To cover the entire possible
$\delta_{CP}$ range, a combined efficiency factor $\eta^*$ corresponding to
multiple sets of detection parameters with different neutrino beam energies and
distances is proposed. The combined efficiency factors $\eta^*$ of various
major experiments are also presented.
