For purposes of subparagraph (i) of this paragraph, the estimated tax is to be paid in equal installments.

(b) If the requirement to pay estimated tax in order to avoid the addition to tax referred to in subdivision (a) of this section is met after the first day of the fourth month but before the second day of the sixth month of the short taxable year, the installment due dates are: the 15th day of the sixth month of such taxable year, the 15th day of the ninth month of such taxable year (unless the short taxable year closed prior to such ninth month, in which case the installment will be eliminated) and the 15th day of the first month of the succeeding taxable year.

(c) If the requirement to pay estimated tax in order to avoid the addition to tax referred to in subdivision (a) of this section is met after the first day of the sixth month but before the second day of the ninth month of the short taxable year, the installment due dates are: the 15th day of the ninth month of such taxable year and the 15th day of the first month of the succeeding taxable year.

(d) If, however, the period for which installments of estimated tax are due is:

(1) at least four months but less than six months;

(2) at least six months but less than nine months and the requirement to pay estimated tax in order to avoid the addition to tax referred to in subdivision (a) of this section is not met until after the first day of the fourth month; or

(3) one of nine months or more and such requirement is not met until after the first day of the sixth month, the installment of estimated tax may be paid in one installment on or before the 15th day of the succeeding taxable year. Assume an individual's short taxable year is the period of 10 months from January 1, 1990 to October 31, 1990 and such individual met the requirement to pay estimated tax in order to avoid the addition to tax referred to in subdivision (a) of this section on or before April 1, 1990. Such estimated tax is payable in four equal installments: due on April 15th, June 15th, September 15th and November 15th. Assume the same facts as in example 1, except that the requirement to pay estimated tax in order to avoid the addition to tax referred to in subdivision (a) of this section was met on April 30, 1990. Such estimated tax is payable in three equal installments: due on June 15th, September 15th and November 15th.