I tweaked the retreat values, made them a little random (so a big stack of retreating units may feature some units going to different adjacent regions), and made it displayed so the player can see it and know what is going on.
Each region has a base retreat value, plus a random value (a "die" with the same number of sides as the base value). E.g. "Retreat value: 7+d7" means that each unit, individually, as it retreats will evaluate a random number between 8 and 14 (inclusive) for that region, and similarly for all adjacent regions, and retreat to the region with the largest result.
The randomness involved is not as large as it looks with the X + dX value. I've found that unless the base values are pretty close most units really go the the larger base value retreat region.
Components in the retreat value are rail, factories, and friendly population of the region, PLUS all those factors for regions adjacent to the region, PLUS supplies in the region.
So, the player can influence retreats directly by placing supply. 1 supply --> 1 base retreat point.
