Disruption during movement.

[krmiller]

From my understanding (the manual is not helpfull at all): 

Every time a unit gets hit by enemy opportunity fire (when you move it in your turn), it checks whether it needs to pass a morale test based on the casualties suffered versus its unit size (formula below). The more casualties suffered and the smaller the unit, the more likely it is forced to conduct a morale test. If it does need to undergo a morale test and it fails the test, the unit gets disrupted. If it passes, nothing happens.

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Shock test formula

casualties suffered* / (   casualties suffered* + unit strength**/10   )

* for big test: accumulated casualties during last enemy turn, for small test: casualties from the opportunity fire action
** this is capped at 250, i.e. units smaller than 250 men still count as having a strength of 250; this mainly helps cavalry and skirmishers
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It looks as if you are quoting the Napoleonic Users Manual here.
I'm not as familiar with it but I checked and yes those games use unit strength to determine if Morale Check is required.
According to the manuals the ACW games use only casualties caused in the current combat.
loss / (loss + 25)
This check is used whether the combat is Fire or Melee.
According to the ACW Manual the only time a unit is automatically required to check morale is if the
1. Defender in a melee loses the melee and has to retreat.
2.  Unit is at Max Fatigue.