The method described in article 40.2 of the LIS

calculation basis will be taken as the full payment for the last tax period whose declaration deadline expired on the 1st of April, October and December, less deductions and bonuses, as well as in the withholdings and payments on account corresponding to that period, provided that said tax period has been of annual duration.

The percentage of 18 percent, will be applied to said calculation base, resulting in the amount of the corresponding installment payment.

When the last tax period for which the declaration period has expired is shorter than year , the proportional part of the quota from previous tax periods, until completing a twelve-month period.