  We study the problem of EB-leakage that is associated with incomplete
polarized CMB sky. In the blind case that assumes no additional information
about the statistical properties and amplitudes of the signal from the missing
sky region, we prove that the recycling method (Liu et al.~2018) gives the
unique best estimate of the EB-leakage. Compared to the previous method, this
method reduces the uncertainties in the BB power spectrum due to EB-leakage by
more than one order of magnitude in the most interesting domain of multipoles,
where $\ell$ is between $80$ and $200$. This work also provides a useful
guideline for observational design of future CMB experiments.
