in “Domains at War” and the new ACKS Rules “Imperial Imprint” there are rules for supplying Armies (Domains at War Chapter 3).
These rules assume that you supply your army by having a line of supply to your supply base.
Reading alot in the internets, I found a blog of an historian, who described the supply of armies in great detail. His argument is, that supplying armies by wagon over long distances is not possible. After a few days, the draught animals eat more than they are able to carry.
So in the absence of ships, foraging is the only option, larger armies have.
As your rules strive for internal logic and ecomonics that make sense, I would love if you could incorpoerate these calculations in your rules (although this request probably comes two years to late).
Additionally I would love to read about the impact of magic and fantastical creatures to logistics.
Here the links to the relevant blog posts (which are good to read for non-historians like me and explain the topic in great detail and with lots of numbers):