Great calculation!

Just a comment: there's more galley/lav space per person in F than in J than in Y, and this is not taken into account in your area calculation.

Also, to get to a proxy for costs, you need to gross them to take into consideration the fact that on long-hauls there's more labor per passenger in F than in J than in Y, and catering costs (which, however, are quite negligible as a % of cost) also vary by cabin.