  It is shown that expressions for the global Lorentz force associated with a
flaring active region derived by Fisher {\it et al.} (2012) can be used to
estimate the Lorentz force changes for strong fields in large structures over
photospheric subdomains within active regions. Gary's~(2001) model for the
stratified solar atmosphere is used to demonstrate that in large-scale
structures with typical horizontal magnetic length scale $\gg 300$~km and with
strong magnetic fields ($\ge 1$~kG at the $\tau =1$ opacity layer at 5000~\AA),
the Lorentz force acting on the photosphere may be approximated by a surface
integral based on photospheric boundary data alone. These conditions cover many
of the sunspot fields and major neutral lines that have been studied using
Fisher {\it et al.}'s (2012) expressions over the past few years. The method
gives a reasonable estimate of flare-related Lorentz force changes based on
photospheric magnetogram observations provided that the Lorentz force changes
associated with the flare have a lasting effect on the observed fields, and are
not immediately erased by post-flare equilibration processes.
