You don't have to figure out the amount. The fact that the Box 2a is blank on your Form 1099-R does not mean you'll be taxed om the whole amount of your distribution. It could or could not be taxable, it depends on the distribution codes and other factors, like your gross income, filing status etc. Please, don't enter anything to that field if it is blank on your form, it will creates errors in your tax return.
