From: Birkerod, Denmark
15.11. Retreat Attrition
When a unit retreats or displaces, it suffers retreat attrition, which can result in some of its ground elements becoming damaged, destroyed or captured. The extent of retreat attrition is based on the unit’s current morale and the experience and fatigue of the unit’s ground elements. Units with higher morale and ground elements with higher experience and lower fatigue will suffer less from retreat attrition. Damaged ground elements are even more likely to be captured, dependent on their experience and whether the unit has a support squad ground element shortage. Damaged ground elements can also have their equipment destroyed while the manpower in the ground element is classified as disabled. Generic organic vehicles can be damaged or destroyed as a result of unit retreat attrition. Units that are forced to retreat across a river hexside will suffer double the normal retreat attrition for a minor river and triple the retreat attrition for a major river.
If the retreat results in overstacking, the surplus units will do it all over again until they reach a hex where they can stay.
So no, the penalties aren't predictable as in a unit will lose X% of their strength.
The penalties are the same no matter how large the overstacking is, however if 3 units retreat onto 2 units, only 2 of the original units wil continue the retreat. Or in other words, stacking is dynamic during the retreat.
And now one of the more experienced players will post exactly why I'm spectacularly wrong.
"Something is always wrong, Baldrick. The fact that I'm not a millionaire aristocrat with the sexual capacity of a rutting rhino is a constant niggle"
- Edmund Blackadder