Ok.. Well,
1. Im going to have my sub mounted in a way where it is out of the port.
2. The box dimensions are external dimensions.
3. I am using the bottom of the box as part of the port, but i thought i took that into consideration when i was calculating the port displacement. Instead of adding 1.5" to the port width to find the external dimensions of the port, I only added 0.75" since there is only really one side being added to the inside of the box. (Im not sure extactly what end correction means.)
4. Also not 100% sure what you mean by measuring it from the centerline.
(Im thinking iv messed up now ;P)