In Tavis's recent playtests, some confusion arose between the interplay of supply lines moving along multiple rivers. In an attempt to address the variables that might occur in measuring length of supply lines, I've revised the rules. Please let me know if you think this is easier to use than the current rules.
Determining whether a supply line is overextended is a simple matter. Just count the number of 6-mile hexes between the army and its supply base. Count each barren or desert hex as four hexes. Count each jungle, mountain, or swamp hex counts as two hexes. Count every two hill or woods hexes count as three hexes. Count every four road or three settled hexes as just one hex, and do not count hexes where the route runs along a navigable waterway at all! If the total count exceeds 16 hexes (96 miles), the line of supply is overextended.
An army with an overextended supply line is out of supply until it shortens its supply line to within the permitted length.
EXAMPLE #1: Marcus’s army departs from Cyfaraun, marching along an old imperial road for 32 hexes (192 miles). Every 4 road hex counts as 1 hex, so the length of the supply line is (32/4) 8 hexes. The army’s supply line is not overextended.
Marcus’s army then enters the harsh desert of the Waste, travelling 3 hexes. Each desert hex counts as 4 hexes, so the length of the army’s supply line is now (32 / 4) 8 hexes from the road and (3 x 4) 12 hexes from the desert, for a total of 20 hexes. Its supply line is overextended, so Marcus’s army is out of supply!
EXAMPLE #2: Baal the Terrible’s army departs its supply base in Zidium and marches 30 hexes (320 miles) along the coast of Celdorea, and then 3 hexes inland across settled terrain towards Dappakosea. Since every 3 settled hexes count as 1 hex, and hexes where the route runs along a navigable waterway are not counted at all, Baal’s army’s supply line only counts as 1 hex long.
For purposes of measuring their line of supply, elves treat forest terrain as settled, while dwarves treat hills and mountains as settled. Beastmen, who have little need for food or drink, eat anything, and forage rapaciously, treat all terrain as settled.