I know this isn't an original post but I am going to figure out how these mechanics work and hopefully come up with some concrete numbers/equations. First up and the easier of the two to figure out is Cover fire. I did some math but I'd like you guys to confirm for me.

Your top 10 attack stat cards that are not in your attack crew, add them up and divide it by 20. Does this equal your attack cover fire? Please post if it does or does not

Yes, your ATK and DEF cover fire is 5% of the top 10 pirates not in the respective ATK or DEF crew.

For your viewing pleasure:
http://piratemaidens.wikia.com/wiki/Cannons_detail