The method described in article 40.2 of the LIS

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

The percentage of 18 percent will be applied to said calculation base, obtaining as a result the amount of the corresponding fractional payment.

When the last tax period for which the declaration deadline has expired is of duration less than year , the proportional part of the quota from previous tax periods must also be taken into account, until completing a period of twelve months.