If, after paying any installment of estimated tax required by subdivision (b) of Section 19025, the taxpayer makes a new estimate, the amount of each remaining installment (if any) shall be the amount which would have been payable if the new estimate had been made when the first estimate for the taxable year was made, increased or decreased (as the case may be) by the amount computed by dividing—

1. (a) The difference between—

   1. (1) The amount of estimated tax required to be paid before the date on which the new estimate is made, and
   2. (2) The amount of estimated tax which would have been required to be paid before that date if the new estimate had been made when the first estimate was made, by
2. (b) The number of installments remaining to be paid on or after the date on which the new estimate is made.