  The polar precursor method is widely considered to be the most robust
physically motivated method to predict the amplitude of an upcoming solar
cycle.It uses indicators of the magnetic field concentrated near the poles
around sunspot minimum. Here, we present an extensive performance analysis of
various such predictors, based on both observational data (WSO magnetograms,
MWO polar faculae counts and Pulkovo $A(t)$ index) and outputs (polar cap
magnetic flux and global dipole moment) of various existing flux transport
dynamo models.We calculate Pearson correlation coefficients ($r$) of the
predictors with the next cycle amplitude as a function of time measured from
several solar cycle landmarks: setting $r= 0.8$ as a lower limit for acceptable
predictions, we find that observations and models alike indicate that the
earliest time when the polar predictor can be safely used is 4 years after
polar field reversal. This is typically 2--3 years before solar minimum and
about 7~years before the predicted maximum, considerably extending the {usual}
temporal scope of the polar precursor method. Re-evaluating the predictors
another 3 years later, at the time of solar minimum, further increases the
correlation level to $r\ga 0.9$. As an illustration of the result, we determine
the predicted amplitude of Cycle 25 based on the value of the WSO polar field
at the now official minimum date of December 2019 as $126\pm 3$. A forecast
based on the value in early 2017, 4~years after polar reversal would have only
differed from this final prediction by $3.1\pm 14.7$\%.
