Have been testing this supply movement a bit, was checking how to move them with transport planes.
Anyway from what I have seen, I sat down an figured out how much a brigade in general would need in in supplies and an air unit, so if I say 1 supply for each brigade and 3 for each air unit I should be more or less safe in regard to having enough.
I have a little invasion of eastern part of south Africa going on in my game and got about 21 brigades down there, got a lvl 3 port shiping in 12 supplies each day, so my army does not have enough supplies. The thing is its annoying watching it build up the 30 day supply for the 3 divisions down there that get supplies and the rest can just starv. It does not help if I change who should get reinforcement/upgrades first it will still send supplies to the same divisions.
I do agree that it should be possible to ship more supplies via a port, 4 supplies for each level of the port is not enough. My suggestion is double it or say (lvl of port times lvl of port) + 4, so a lvl 1 would get 5 supplies a lvl 5 29 and a 10 would get 104 supplies. Would also like if you could ship supplies to a coastle province 1 or 2 supplies each.
Just my 2 cents on that.