Width of Book Boards

I've always had a problem figuring out the width of a book board. Then I found this nifty definition:

The width is the width of the book block plus the square and minus two board thicknesses to create the hinge.

Works for me. A 2 mm book board has a 4 mm hinge. Although I have also read the hinge should be 3.5 times the thickness; making it a 7 mm hinge.

If the square is 2 mm:

Book block + 2 - 4 = book block - 2.