After examining a few more combat odds, I'm going to amend my best guess formula to the following:
Average damage done by Unit A = Modified Strength A / (Modified Strength A + Modified Strength B) * Damage Factor
Where Damaga factor = 5 + .8 * Hit Points remaining A
That means that a unit with full strength would have a damage factor of 13 and one with 1 life left would have one of 5.8. At half life, the damage factor would be 9.
The Japanese would always have a damage factor of 13.
Additionally, it seems that the damage factor is different (a bit lower) for ranged attacks.
Average damage done by Unit A = Modified Strength A / (Modified Strength A + Modified Strength B) * Damage Factor
Where Damaga factor = 5 + .8 * Hit Points remaining A
That means that a unit with full strength would have a damage factor of 13 and one with 1 life left would have one of 5.8. At half life, the damage factor would be 9.
The Japanese would always have a damage factor of 13.
Additionally, it seems that the damage factor is different (a bit lower) for ranged attacks.