ports through rear deck?

I am wanting to have my port or ports come up through my rear deck, but im wondering what effect part of the port sticking out from the enclosure has on the recommended length/size of the port?

I have done the calculation to figure out the dimensions of the port needed but if say 3" of length is basically outside the enclosure to meet up with the deck do i add 3" to the overall length or does the port just not go as far into the box?

Hope this makes sense.............

Port length is port length and is the same no matter if it's inside or outside the enclosure.

Why do you want the ports extended through the rear deck?  Do you think this will make it louder, or do you simply want the "look?"

So what did you end up doing?  My buddy had the same problem.  Has a '96 Impala and all of his bass was trapped inside the trunk with hardly anything inside the cabin.  He extended ports to the rear deck and that definitely made a huge difference.  Any pix of your mod?