I thought it was tied to the stat favored by the class's tool? Str, Vit and Dex are craftmanship and Int, Mnd and Pie are magic craftmanship.

Alchemist: (MH/OH) Int/Pie
Armorer: Vit/Str
Blacksmith: Str/Mnd
Carpenter: Vit/Dex
Culinarian: Mnd/Pie
Goldsmith: Dex/Int
Leatherworker: Vit/Int
Weaver: Dex/Mnd

I bet it's more complicated than that though. I bet there are a lot of interdependancies worked in there too. Just looking at how many different jobs can use the same training manual, lets us know the answer isn't simple or cut and dry. It's probably some percentage based factoring in everything including the crystals. Maybe some synths are 705/30% favoring magic and some maybe 90%/10% favoring craftsmanship.