If we want all the slices to be the same length so they fit in the box well we have to trim the brisket to the width we want before cooking. In Texas you are not allowed to turn in slices with trimmed or broken ends, That is an automatic DQ. That being said, it isn't required that the pieces fit just so. We can carve 10" slices and lay them in the box curved around.
Here is one way to do it. Trim the brisket between 9" and 10" wide with the grain making sure your thickest part of the flat is in the center. It looks weird but after cooking it will produce a very uniform slice.