During wartime, the probability centers around supply line efficiency - if the unit is best supplied by a city 10 hexes away that has a maximum supply line length of 20, then it is using 50% of the supply line length, therefore Supple Line Efficiency = 50%. If the unit is in an enemy hex, this number is cut in half - meaning the unit has a 25% chance of movement impact; it also has a 25% chance of not being able to attack; and finally a 25% chance of taking Attrition "damage." Three shots at impact @25% chance each.
If the unit has adjacent enemy units or is next to an enemy city, the probability increases 5% per unit and 5% for the city. If a unit was surrounded by enemies and was next to an enemy city, it would add 35% to this basic chance of Attrition - or 60% in the previous example - 60% for each of the 3 possible effects. This sounds pretty sound to me.
On neutral territory during wartime, those numbers are half as much - 12.5% for the basic unit using half the length of the supply line from the best supplying city. This feels pretty good to me, too.
Of course, all of this gets heavily impacted if the civ has too many units. Basically, the mod adds 5% for each excessive unit - up to a limit which depends on the plot relationship (on a friendly hex, max is 20%, neutral = 30%, and enemy = 35%) - these numbers are added to the basic probability of impact. It basically demonstrates that when a civ has more units than it can efficiently supply, those effects are felt by every unit in the system, which again, feels right to me.
A few of my earlier test games saw inordinate amount of Attrition events, so I was wondering if it should be toned down. However, when I started running the 500+ turn games, I wasn't really seeing many events at all. This should be expected as most of the game turns involve units on friendly territory and not too many excessive units. In those circumstances, we shouldn't experience much attrition at all. But I am seeing occasional events spread out through all the civs, suggesting the mod is working about like it should. I think....
So, I guess it's a long way of saying, let's test this version and see how it does. I'm particularly interested in help deciding what the best numbers are for the game's normal supply formula - and how well the mod plays with other mods.