Apple and Oranges
I think:
Your statements are correct (enough).
But the positions you are deriving like that are probably not what you are trying to find.
Subtly different but aren't you trying to find the:
1 Station of the empty seat
2 Station of a person when seated BUT NOT with a seat attached to him.
Your Station of person plus seat is not giving you the station of the person.
I presume you want to find the station of the person - so that you can then apply the assumption that any person sitting in the seat is at that station.
The 'person plus seat' station will lie between the seat station and the person station.
Question:
Does the 'Standard passenger' (male/european/onshore/shorthaul - for they are all different would you believe?) have a standard CG location.
(Just curious.... but does it actually matter?)