(a) For the purposes of Section 19142, the amount of the underpayment shall be the excess of the amount calculated in paragraph (1) over the amount calculated in paragraph (2).

1. (1)

   1. (A) The amount of the installment that would be required to be paid if the estimated tax were equal to the applicable percentage of the tax shown on the return for the taxable year.
   2. (B) If no return was filed, the applicable percentage of the tax for that year.
2. (2) The amount, if any, of the installment paid on or before the last date prescribed for payment.

(b) For purposes of this section, the “applicable percentage” shall be as follows:

1. (1) For taxable years beginning before January 1, 1998, 95 percent.
2. (2) For taxable years beginning on or after January 1, 1998, 100 percent.