I've not done it this way as I trim pretty aggressively from all sides, but I understand what you are saying... If you only cut straight up/down you mightl end up making the brisket narrower but only really end up potentially trimming small portions of the flat slices that will go in the box.
You really have to look at each brisket differently and assess which way the grain in running and ideally where your slices are going to come from..
Then.. if you want to be like Sledneck, bust out a 1" x 30' Stanley Fat Max tape measure and find the sweet spot picture frame section and measure approximately the width of that area and cut it down to approximately the size of the turn in box width...
if you do this, just be sure to account for and borders you may have in the box from garnish, and if you position slices horizontally or diagonally and