Your system seem a tad out of "ionic balance", but not by much. The first thing you need to raise is your mag, that is waaaay to low. Try to raise it (showily) to 1300-1350 and keep it there, it will make your CAL/ALK much easier to balance and keep stabilized.
With that being said, the ionic balance for a CAL reading of 460 is right around 14 for ALK. I would try to aim for 420 cal and 8 to 8.5 alk.
Read more on this here
http://reefkeeping.com/issues/2002-0...ture/index.php scroll down to the balance table.
Have you tried a different test kit to make 100% sure it is indeed at 16? Maybe borrow a friends?