This will show you why
On an outside corner, you would need to take 1/2" off one of the boards.
As for the panel widths, I would suggest that you divide the wall into a size that you feel comfortable with. Since every one of my walls were a different length, they are similar to each other, but far from exact.
Furniture placement will camouflage a lot of this.
Here are the specs for the top of section 2
These are the trim pieces that I used
Some of the hard cuts that Joe and Mike did
Wrapped the corners