Inches are evil. Fractional evil, but pure evil.

OK, the page trim is 5.5 inches in width, the left and right margins are each six-eighths of an inch, and there’s a quarter-inch gutter—quick, what is the text width?

On the other hand, if the trim width is 140 millimeters, the right and left margins are 20 millimeters each (140 – 40) and the gutter is set to 6 millimeters—quick, the text width is 94 millimeters. Piece of soup.