Try "W/D Opening Surface on the Side Opposite to the Reveal Side". This will get you closer, but still not exact.
EDIT: It may be worth mentioning that the Cadimage doors/windows have a "Total Area" parameter that calculates the area of the unit, and is reasonably accurate. I don't know why GS doesn't have such a parameter.
Richard
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Richard Morrison, Architect-Interior Designer
AC26 (since AC6.0), Win10