Welcome Blacksails,
For the gunnery table you apply all modifiers.
So for an abeam escort look up that column on the table.
Apply the shift for the Targetting Matrix.
Apply the shift for the blastmarkers
Apply a shift for range if needed.
The order in which the shifts are applied doesn't matter.
And the most right column is the lowest, so further right shifts are negated. And the same on the left side.
So, yeah, you are overthinking it.