  We describe an update to the Herschel-SPIRE Fourier-Transform Spectrometer
(FTS) calibration for extended sources, which incorporates a correction for the
frequency-dependent far-field feedhorn efficiency, $\eta_\mathrm{FF}$. This
significant correction affects all FTS extended-source calibrated spectra in
sparse or mapping mode, regardless of the spectral resolution. Line fluxes and
continuum levels are underestimated by factors of 1.3-2 in the Spectrometer
Long-Wavelength band (SLW, 447-1018 GHz; 671-294 $\mu$m) and 1.4-1.5 in the
Spectrometer Short-Wavelength band (SSW, 944-1568 GHz; 318-191 $\mu$m). The
correction was implemented in the FTS pipeline version 14.1 and has also been
described in the SPIRE Handbook since Feb 2017. Studies based on
extended-source calibrated spectra produced prior to this pipeline version
should be critically reconsidered using the current products available in the
Herschel Science Archive. Once the extended-source calibrated spectra are
corrected for $\eta_\mathrm{FF}$, the synthetic photometry and the broadband
intensities from SPIRE photometer maps agree within 2-4% -- similar levels to
the comparison of point-source calibrated spectra and photometry from
point-source calibrated maps. The two calibration schemes for the FTS are now
self-consistent: the conversion between the corrected extended-source and
point-source calibrated spectra can be achieved with the beam solid angle and a
gain correction that accounts for the diffraction loss.
