You don't need it for extracting files, but for importing..
yy is the size. ((Size in kb x 1024)/100)+1 in decimal then convert to hex.
For example:
123KB file
123 x 1024 = 125952
125952 divided by 100 = 1259.52
1259.52 + 1 = 1260.52
Convert 1260.52 to hexadecimal = 4EC
If it is not 4...