Your zeroing theory is based on fuzzy logic scooby and Mr E. The mechanical centreline of the Bren barrel is 1" to the RIGHT of the optical axis of the sights. Optical axis always travel in straight lines - as is the mechanical axis of the bore of the Bren......, that is the vertical path of the bullet too.... VERTICAL don't forget.
If the optical line of sight converges with the mechanical axis of the bore at a distance of, say, 100 or 200 yards, then at 400 yards you won't even be hitting the target. Take it from me, based on my limited experience with Brens and optro/mechanical axis! Zero in properly using the tried and trusted EMER formula. It works reasonably well for the British, Australian
, Canadian
, New Zealand, South African, Indian and 30 or so other user nations for 60+ years now.
Frustrating or what.........