I would prefer individual trade codes, however. If I knew enough about the ancient ports to make them individual and unique. Instead, I might have a few unique ports (Alexandria, Athens, Antioch, Ephesus, etc.) then use zones for the other ports, the Levant, Greece in general, the Nile, etc.
I'm all for your project and will adopt it as soon as it's completed!
Quite! Can we note the primary biome (or two) of each city, which become DMs to each type of trade good, similar to Book 2?
The "terrain types" in LBB3 are close-enough matches to the various "biome" lists that I'll try using them directly. So the LBB3-biomes might be:
Alexandria: Desert, Beach
Athens: Rough, Forest
Antioch: Rough, Prairie
Ephesus: River
Add a few more trade codes into the mix (Cp, Sea (i.e. Seaport), In, Ni, Ag, Na, Ri, Po) and we may have something. Offhand, I'd wild-guess:
Alexandria: Cp De Sea In Na Ri
Athens: Cp Rough Forest Ni Na Ri
Antioch: Cp Rough Prairie Ni Ag Ri
Ephesus: River In Ag
So for instance, Fish Sauce DMs might include +1 for Seaports, +1 for River, and +1 for In. Ephesus therefore would have a Fish Sauce DM of +2 for production (i.e. lot size), and -2 for demand (i.e. cost). Would that work?
This would also work fine when defining trade zones: you simply apply a set of trade codes to all settlements within a region, unless they have their own codes.