Auto defensive fire

[tbridges]

Does anyone know how auto defensive fire is calculated?

Is the total number of defensive shots figured per hex or per unit? Do defenders get three shots per hex or three shots per unit in their stack? If offensive units are attacking from three different hexes can the defenders fire up to three shots at each attacker's hex, or can each defender in the hex fire three times at each attacker in multiple hexes? It seems that when I fire my offensive unit I sometimes get no defensive fire at all and sometimes I get a veritable hail of return fire.

I just don't understand this at all, there seems to be no rhyme or reason to it. Surely there is more to this than just the roll of a die.