To ensure correct calculations, the "Monthly table" checkbox must be ticked in the withholding tax tab of the employee's salary data. Indicate the employee's full-time rate of employment. Then, as for part-time monthly salaries, indicate whether the salary should be brought up to a different occupancy rate based on a possible other job, in order to find the tax rate to be applied:

Occupancy rates are stored each month by the program at closing, and can be viewed at Data diagnostics / Tools / Period and degrees of occupancy  / Update occupancy rate / History of Occupancy rates :

(If the salary is to be traced back to another occupation rate  for tax purposes, there should be a figure in the last column.)