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Observations of two astronomical targets with the G800L+POLV polarizers are required for base calibrationdoublePoint one unpolarized (null) standard to measure any instrumental polarization effects one object of known polarization characteristics to measure the polarization efficiency coefficient comma and establish the absolute polarization scale and the polarization position angle fiducial point. This is part III of the campaign comma in which the unpolarized standard is observed at a spacecraft roll angle that is virgul135 degrees rotation from the very first observation of the unpolarized standard. This helps to remove a degeneracy in the instrumentlal polarization position angle. The unpolarized white dwarf G191-B2B is our primary null calibrator for the ACS comma and has a long legacy of observations using the WFC and HRC for imaging polarimetry. This heritage ensures continuity between the spectro- and imaging polarimetry. It will assist in cross-calibration of the F775W+POLV combination comma due to concerns about the polarimetry ground test data beyond 0.8 microns (see comma e.g. comma Walsh 2001 comma ACS ISR 2001-01).