From: Leeuwarden, Netherlands
I know the supply thing can de dazzling, but it makes totally sense once you've grasped it.
Like Twotribes here I usually play on large or very large maps and I almost never use more than Supreme HQ (which stays in your starting city and gets all the supply...well there are exceptions to that rule, but in general the SHQ gets all the supply) and Division HQ's.
First thing is build roads with your engineers, no matter how hard the terrain is, if there are roads/railroads, the supply flow is 100%. Oh build bridges too, very important for your supply flow, makes sense no?
Matter of fact I don't use the 250AP thingy or the "supply layer" at all. Simple rule is your ever expanding empire and with your troops ever farther away from their supply source (the SHQ) they *need* to be connected via a railroad system. To get the supply flowing, and to get fresh reinforcements (via strategic transport). You can use the supply layer thing to get an idea of where and how much supply is flowing though, ofcourse that layer will change the more rr you build. Heck sometimes you *have* to build a railroad to an isolated unit. Happens to me sometimes, send out a cavalry unit to capture something, and then that unit runs out of supply and can't move (or attack for that matter) anymore.
But it comes to you naturally after a while because supply flows as it does in real life, via roads and railroads. Units a couple of hexes away from your supply grid get 100%, imagine trucks loaded with beans and bullets moving along dirtroads. They'll be fine within a few hexes. Now add even more hexes away from the supply grid, now the trucks have more difficulty reaching your troops. Now add even more hexes and mud, or hills and forests...*now* your trucks have major problems reaching your unit. Until they can't even reach your units anymore, say Russian rasputitsa style, not good.
So yeah, build roads, that's it really.