16a just refers to the first state row for boxes 14,15,16, because people who move states during the year will sometimes have two rows showing..
Anyhow, if box 2a has a non-zero value in it, copy box 2a into box 16.
If box 2a is empty, or marked as not determined in box 2b, you can use the value of box 1, as long as you didn't make any after-tax contributions to the source of those funds.