You called me out. You caught me away from my library so mind this won't be exact but it will be really close.

Zone 0....0%
Zone I.....1.5%
Zone II....3%
Zone III...6%
Zone IV...10%
Zone V....18%
Zone VI...26%
Zone VII..36%
Zone VIII.50%
Zone IX...68%
Zone X....100%

Mind you a freehand curve on a hand mand eleven point x and y graph but this is real close to the numbers. Never committed the exact numbers to memory but this will put you there. The % numbers regard percentage of light reflected at that zone. And zones 0 and X are probaly a couple of points from the extreme values here so it might be SLIGHTLY skewed, like I said, close not perfect. But very close IIRC.