Ranged Weapon protocols

In ACKS and similar games, firing ranged weapons at engaged melee combatants is not permitted.

How do you adjudicate firing past melee combatants at non-engaged characters/monsters beyond the engaged 1st and 2nd ranks? (I'm assuming 2nd rank spear carriers are also considered engaged here - maybe that's a separate query). Do you take cues from the cover guidelines? (p 103)

I was thinking a -1 "cover" penalty for each rank of allies and foes might be appropriate to the typical situation in a dungeon. 

Example 1: [Archers & Mage][Spears][Front Line]<> [Orcs Front Line][Orc Spears][Orc Shaman & Orc Archers] = a - 4 cover penalty for the Archers to fire at the Orc Shaman & a -4 cover penalty for the Orc Archers to fire at the Mage.

Example 2: [Archers & Mage][Front Line]<>[Orc Front Line][Orc Shaman & Orc Archers] = a -2 cover penalty for the Archer to fire at the Orc Shaman & a -2 cover penalty for the Orc Archers to fire at the Mage.

For these examples, assume a 10' wide passage with each rank immediately behind the other. And maybe it caps at -4 and gets no worse regardless? (equivalent to blindness or striking an opponent through a small window). Or maybe you count every rank and it could go as high as a -8? ACKS games tend to have quite a troupe. Or maybe it's always a -4 to fire past friends and/or foes regardless of how many or how few ranks? 

This seems a critical thing to determine as it impacts PC spell-casters and monster spell-casters the PCs might seek to hit/disrupt. Too severe a penalty and you've got third rank PCs often with not much to do. Too light a penalty and you've got orc crossbowman dropping the pointy-hat robed folks all day long. Given their low ACs and the threat of that Sleep spell they seem to be casting,that's not hard to imagine even at a flat -4 come to think of it...

Or maybe Judges often rule no shot is available due to cramped conditions? Regardless of the above, do you incorporate friendly fire house rules?

I'm no Autarch, but I generally assume that firing through a melee is just as hard, or harder, than firing at a specific creature in that melee. After all, the reason why hitting a specific creature in a melee is impossible is that they're moving around all the time, making it impossible to guarantee you'll hit the correct target; Firing between such combatants to hit someone on the other side, obscured by even more moving characters, and who's not engaged in melee and is therefore in a better position to watch for ranged threats, should be even harder than impossible.

Of course, for a fight in the open air or in a chamber with a very high ceiling, it's possible to fire in an arc, which makes firing over the heads of melee combatants more viable. I generally allow firing at targets on the other side of the melee in such cases.

Something that's never come up in my campaign (because my players have a healthy sense of their own mortality) is for a character to accept the risk of hitting an ally when firing into melee, and doing it anyway. I've never really thought about what to do in that case. I guess some variant of your cover rules might be appropriate - but if an arrow ever missed due to cover, I'd take it as meaning that the arrow had hit said cover, either penetrating it or glancing off its armour. To determine which, I guess I'd make an attack throw of 1+, applying the cover-providing character's armour class to the throw number... Or something like that.

Thanks for the response, GMJoe. Some refined searching led me to this previous thread which I also found helpful: