I don't know if this is what you're looking for
BLOCK SIZE WANTED (divided by) CURRENT BLOCK SIZE = PERCENT TO ENLARGE OR REDUCE
Example:
To enlarge: Size you want = 12" and Current size = 4"
12/4=3 (answer times 100 = 300%)
To reduce: Size you want = 7" and Current size = 12"
7/12=.58 (answer times 100 = 58%)