Okay. If you start with a charm square (which is 5 inches square) and they finish to 4.5 inches. You will need 12 charm squares to equal the 54 inches wide. Then you will need 14 to equal 63 inches long (60 inches ends up with .333 of a charm square). So if you multiply 12 times 14 you get 168 charm squares.
Holpe this helps.