The resulting width measurement will be the vertical piece's finished width. It is cut to this measurement as soon as it is determined that the piece of stock is of sufficient length. We will use the following abbreviations to show that the measurements now include the horizontal piece:

Width of the book, enclosed in the horizontal piece: Wh
Thickness of the book, enclosed in the horizontal piece: Th

In our example, we will make a vertical piece with two full-length flaps, just for the sake of variation. We could make it with two half-length flaps like the horizontal piece, though we would want the vertical flaps to be on the inside, rather than on the outside as in our example.

So the formula for the minimum length of the piece of stock we need is:

3L + 2Th + 1mm

(The 1mm at the end is to compensate for the fact that one of the flaps will have to fold over the other one.)

Once we are certain that our piece of stock is long enough, it is cut to a width of Wh. The length is left excessive.