  I show that the standard microlensing technique to measure the angular radius
of a star using color/surface-brightness relations can be inverted, via
late-time proper motion measurements, to calibrate these relations. The method
is especially useful for very metal-rich stars because such stars are in short
supply in the solar neighborhood where other methods are most effective, but
very abundant in Galactic bulge microlensing fields. I provide a list of eight
spectroscopically identified high-metallicity bulge stars with the requisite
finite-source effects, seven of which will be suitable calibrators when the
Giant Magellan Telescope comes on line. Many more such sources can be extracted
from current and future microlensing surveys.
