major confusion over port size happening here...

greetingz

My Kenwood amp for the subs arrived today, so it`s wiring-up day and sub-box making day woo hoo!

A few questions:

With sub boxes (which can be really complicated): Kenwood stopped making my subs, so I had to search online for the recommended box dimensions. All that I could find was this post referring to a 1.6ft^3 box with a 3" wide and 6" long port. However this port calculator says that I need a 5.32" long port, based on my Sub specs, and this one reckons I need a 7.5" long port.

Hence my confusion!
Which would be correct length?
Would it make that much of a difference anyway?
Does it matter where on the box the port goes (like close to a corner / in the middle?)
For 2 subs in one box, do you just double the volume and add another port of the same size?

Also: When connecting the + power lead to the battery, do you need to actually connect to the battery itself, or can you join into another + power lead that`s already inside the car? (I have a proper 8G wiring kit with fuse etc)