Yea sometimes it can look strange, currently I simply have no clue what impacts the triggering of defensive fire except that the OR "Proportional Opportunity Fire" seems to make it more likely for bigger stacks to trigger defensive fire, but otherwise no clue, may simply be randomly but I think there is a good chance that John Tiller made it a bit more complicated, sounds simply too easy for a Math Professor.
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Lieutenant General Christian Hecht
Commander I Corps, Army of the Potomac
"Where to stop? I don't know. At Hell, I expect."